UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 6)

CAPITAL TRUST, INC.

(Name of Issuer)

Class A Common Stock, Par Value $0.01 Per Share

(Title of Class of Securities)

14052H100

(CUSIP Number)

Ira S. Lederman

Senior Vice President - General Counsel and Corporate Secretary

W. R. Berkley Corporation

475 Steamboat Road

Greenwich, Connecticut 06830

(203) 629-3000

With Copies to:

Jeffrey S. Hochman, Esq.

Mark A. Cognetti, Esq.

Willkie Farr & Gallagher LLP

787 Seventh Avenue

New York, New York 10019-6099

(212) 728-8000

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

May 23, 2012

(Date of Event Which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition which is the subject of this Schedule 13D, and is filing this schedule because of Rule 13d-1(e), Rule 13d-1(f) or Rule 13d-1(g), check the following box  ¨.

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB control number.

This Amendment No. 6 to Schedule 13D (“Amendment No. 6”) is being filed on behalf of W. R. Berkley Corporation, a Delaware corporation (“Berkley”), Admiral Insurance Company, a Delaware corporation (“Admiral”), Berkley Insurance Company, a Delaware corporation (“BIC”), Berkley Regional Insurance Company, a Delaware corporation (“BRIC”), and Nautilus Insurance Company, an Arizona corporation (“Nautilus” and, together with Berkley, Admiral, BIC and BRIC, the “Reporting Persons”). Amendment No. 6 relates to shares of Class A Common Stock, par value $0.01 per share (the “Common Stock”), of Capital Trust, Inc., a Maryland corporation (the “Issuer”). Amendment No. 6 amends and supplements the Schedule 13D, dated May 11, 2004 (“Original Schedule 13D”), as filed with the Securities and Exchange Commission (the “Commission”) on May 21, 2004, as amended by Amendment No. 1 to Schedule 13D, dated June 17, 2004, as filed with the Commission on June 21, 2004, Amendment No. 2 to the Schedule 13D, dated September 13, 2004, as filed with the Commission on September 13, 2004, Amendment No. 3 to the Schedule 13D, dated July 10, 2007, as filed with the Commission on July 13, 2007, Amendment No. 4 to the Schedule 13D, dated July 30, 2007, as filed with the Commission on August 1, 2007 and Amendment No. 5 to the Schedule 13D, dated August 6, 2007, as filed with the Commission on August 6, 2007 (together with the Original Schedule 13D, the “Schedule 13D”), in connection with the additional information set forth herein.

Item 3 of the Schedule 13D is hereby amended by the addition of the following information:

The Reporting Persons expect that the total amount of funds used by the Reporting Persons to fund the Proposed Transaction (as defined below) would be from the available cash of one or more of the Reporting Persons.

Item 4 of Schedule 13D is hereby amended by the addition of the following information:

On May 23, 2012, Berkley delivered a non-binding letter (the “Proposal Letter”) to Evercore Partners, a financial advisor retained by the Issuer to review potential strategic options for the Issuer.

As described in the Proposal Letter, Berkley expressed a non-binding indication of interest in a transaction (the “Proposed Transaction”) in which Berkley or one or more of its subsidiaries would (i) acquire (a) all of the equity interests of CT Investment Management Co., LLC (“CTIMCO”), (b) certain equity interests in private equity funds managed by subsidiaries of CTIMCO which interests are held by subsidiaries of the Issuer, and (c) certain preferred shares issued by the issuers of CDOs managed by CTIMCO, which preferred shares are held by subsidiaries of the Issuer, (ii) assume certain cash-based performance awards relating to private equity funds managed by the Issuer and long-term cash-based performance awards relating to the Issuer’s legacy REIT assets or provide holders of such awards with new awards or arrangements with substantially similar economics thereto in lieu of such existing performance awards and (iii) assume the Issuer’s lease for office space in New York, New York, for, subject to a tangible net worth adjustment and an indemnification escrow, total consideration of between $25 million and $30 million, payable in cash.

No assurances can be given that a transaction will be consummated. The Proposal Letter provides that no legally binding obligation with respect to a transaction will exist unless and until mutually acceptable definitive documentation has been executed and delivered with respect thereto. Berkley reserves the right to modify or withdraw the Proposal Letter at any time.

The proposed transaction described herein may result in one or more of the actions specified in clauses (a) through (j) of Item 4 of Schedule 13D, including, without limitation, a sale or transfer of a material amount of assets of the Issuer or any of its subsidiaries, other material changes to the Issuer’s business or corporate structure, or any action similar to those enumerated in Item 4 of Schedule 13D.

The summary of the Proposal Letter, above, is qualified in its entirety by reference to the Proposal Letter, attached as Exhibit 5 hereto, which is incorporated by reference herein.

The Schedule 13D is not an offer to purchase or a solicitation of any offer to sell any securities.

Item 5 of this Schedule 13D is hereby amended and supplemented to add the following:

Rows (7) through (11) and (13) of the cover pages to this Amendment No. 6 are hereby incorporated by reference into this Item 5.

Berkley and the Issuer are parties to a Confidentiality Agreement that includes a standstill provision (the “Standstill”) that provides that, during the twelve-month period commencing on April 1, 2012, except pursuant to a negotiated business transaction between Berkley and the Issuer approved by the board of directors of the Issuer and subject to certain limited exceptions including the making of a proposal in accordance with an invitation by the Issuer, Berkley will not, in any manner, directly or indirectly, acting alone or in concert with others:

(i) make, effect, initiate, cause or participate in (A) any acquisition of beneficial ownership of any securities or debt obligations of the Issuer or any of its subsidiaries, (B) any acquisition of any assets of the Issuer or any of its subsidiaries, (C) any tender offer, exchange offer, merger, business combination, recapitalization, restructuring, liquidation, dissolution or extraordinary transaction involving the Issuer or any of its subsidiaries, or involving any securities or assets of the Issuer or any of its subsidiaries, (D) any solicitation of proxies or consents with respect to any securities of the Issuer or any of its subsidiaries or (E) any acquisition of beneficial ownership of any debt, equity or hybrid instrument, whether in cash or synthetic form, the cash flows or returns of which are derived from any property that serves as collateral for any of the Issuer’s or any of its subsidiaries’ assets or liabilities;

(ii) form, join or participate in any “group” (as defined in the Securities Exchange Act of 1934 and the rules promulgated thereunder) for the purposes of acquiring, holding, voting or disposing of securities or debt obligations of the Issuer or any of its subsidiaries;

(iii) seek to control the management, board of directors or policies of the Issuer or any of its subsidiaries;

(iv) take any action which is reasonably likely to legally require the Issuer or any of its subsidiaries to make a public announcement regarding any of the types of matters set forth in subsection (i), above;

(v) request or propose that the Issuer (or its directors, officers, employees or agents), directly or indirectly, amend or waive any provision of the Standstill; or

(vi) agree or offer to take, knowingly encourage, propose (publicly or otherwise) the taking of, knowingly assist, induce or encourage any other person to take, or enter into discussions or arrangements with any third parties for the purpose of taking, any action referred to above.

Item 7 of Schedule 13D is hereby amended to include the following:

SIGNATURES

After reasonable inquiry and to the best of our knowledge and belief, the undersigned certify that the information set forth in this statement is true, complete and correct.

Exhibit 5

[W. R. Berkley Corporation Letterhead]

May 23, 2012

Evercore Partners

55 East 52^nd Street

New York, NY 10055

Attention: Greg Brooks

Dear Mr. Brooks:

W. R. Berkley Corporation is pleased to submit this confidential, non-binding indication of interest in a transaction pursuant to which W. R. Berkley Corporation or one or more of its subsidiaries (collectively, “W. R. Berkley”) would acquire the investment management business of Capital Trust, Inc. (the “Company”) as further described below (such contemplated transaction, the “Proposal”).

We are prepared to move expeditiously to complete a transaction and, per your bid instruction letter of May 14, 2012, herein provide our Proposal that includes the following key terms:

Transaction Structure and Consideration. Based upon the financial and other information made available to us, W. R. Berkley proposes to (i) acquire (a) all of the issued and outstanding interests of CT Investment Management Co., LLC (“CTIMCO”), (b) the limited partnership investment in CT Opportunity Partners I, L.P. (“CTOPI”) (including undrawn capital commitment with respect thereto) held by a subsidiary of the Company, (c) CTIMCO’s member interest in CT High Grade Partners II, LLC (“CT High Grade”) (including CTIMCO’s undrawn capital commitment with respect thereto) and (d) the preferred shares issued by the issuer of each CDO managed by CTIMCO to the extent held by direct or indirect subsidiaries of the Company and (ii) assume the Company’s lease for office space at 410 Park Avenue, New York, New York 10022. In addition, in connection with the transaction, W.R. Berkley would either assume the existing CTOPI cash-based performance awards and the long-term cash-based performance awards relating to the Company’s legacy REIT assets or provide holders thereof with new awards or arrangements with substantially similar economics thereto in lieu of such existing performance awards. W. R. Berkley proposes to acquire the forgoing for total consideration of between $25 million and $30 million (the “Purchase Price”), payable in cash, subject to the adjustments described below.

W. R. Berkley would be willing to discuss alternatives to this proposed transaction structure which would permit W. R. Berkley to acquire substantially the same assets and be mutually beneficial to W. R. Berkley and the Company from a tax perspective.

Purchase Price Adjustments. As part of the transaction, at closing W. R. Berkley would expect that CTIMCO would have an amount of tangible member’s equity consistent with the financial statements, business plan and projections provided by the Company to W. R. Berkley. Based on information received to date, W. R. Berkley believes that this figure would be $7.5 million. At closing, W. R. Berkley would increase the Purchase Price on a dollar-for-dollar basis by the amount of any excess tangible member’s equity above the mutually agreed upon target tangible member’s equity. Conversely, W. R. Berkley would expect a reduction in the Purchase Price on a dollar-for-dollar basis for any shortfall in the mutually agreed upon target tangible member’s equity.

Additionally, to the extent that the Company funds any portion of its uncalled capital commitment to CTOPI for purposes of funding investments prior to the closing, W. R. Berkley is willing to increase the Purchase Price in an amount equal to the book value of such additional investments made measured consistently with past practice at the time of the closing. To the extent that the Company funds any portion of its uncalled capital commitment to CTOPI for any other purpose consistent with its current funding obligations, W. R. Berkley is willing to increase the Purchase Price by such aggregate amount.

Key Assumptions: This Proposal is based on, among other things, the information the Company has previously provided to W. R. Berkley and certain of W. R. Berkley’s own assumptions, including:

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Management Structure: W. R. Berkley has a history of empowering management teams to operate within a decentralized structure within the W. R. Berkley family and understands the importance of the current CTIMCO management team to the success of its businesses. Given our future objectives for CTIMCO, we would expect to retain the services of CTIMCO’s employees as part of any transaction and operate CTIMCO on a stand-alone basis following the closing of such transaction.

Sources of Financing and Conditions to Closing: W. R. Berkley intends to fully fund the Purchase Price from internally generated funds and thus the closing of the transaction would not be conditioned on the receipt of financing. Consummation of the closing of the transaction would be conditioned on the following:

Definitive Documents: The Proposal will be reflected in a definitive purchase agreement acceptable to W. R. Berkley and the Company and containing representations, warranties, covenants and indemnities customary for a transaction of this nature and an escrow in order to support the Company’s agreed upon indemnification obligations. Additionally, W. R. Berkley expects that certain key employees will agree to employment agreements or long-term incentive arrangements concurrently with the execution of the purchase agreement and the Company will enter into other ancillary documents related to the transactions.

Due Diligence Requirements: W. R. Berkley does not expect that its additional due diligence review of CTIMCO, CTOPI or CT High Grade will unduly delay the execution of a definitive agreement with respect to the Proposal. However, W. R. Berkley reserves the right to finalize its due diligence during the period of negotiation of a definitive agreement, including following its review of the disclosure schedules to be provided in connection therewith.

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Required Approval: Except for the conditions to closing described above, W. R. Berkley does not believe that any additional approvals will be required to consummate the Proposal. Specifically, W. R. Berkley has obtained all requisite internal approvals necessary to proceed with the Proposal.

Timing: We are prepared to move expeditiously to negotiate and execute definitive documents with respect to the Proposal and believe this could be accomplished over the next four weeks. The timing of the closing of the transaction would be dependent upon the timing of the satisfaction of the closing conditions, including the receipt of each necessary regulatory, contract or other approval.

Exclusivity: To the extent that you determine to move forward with the Proposal, and in consideration of the substantial resources we have expended to date and will continue to expend in order to negotiate a definitive agreement, we would expect that you would enter into an appropriate exclusivity arrangement with us of thirty days.

Expenses: Whether or not the transactions contemplated hereby are consummated, W. R. Berkley and the Company shall each pay its own fees and expenses incident to the negotiation, preparation, execution and performance of the Proposal or other transaction agreed to, including attorneys’, accountants’ and other advisors’ fees and the fees and expenses of any agent, broker, finder, investment banker, financial advisor or agent retained by such party in connection therewith.

Non-Binding: This letter constitutes only a general, non-binding preliminary indication of interest on the part of W. R. Berkley and is not intended to, and does not, create a legally binding commitment or any obligation, right or liabilities of any nature whatsoever, on the part of W. R. Berkley or the Company. Unless and until a final definitive agreement providing for the Proposal or other transaction has been executed and delivered, none of W. R. Berkley, the Company or any of their respective affiliates is or will be under any legal obligation of any kind whatsoever with respect to the Proposal or other transaction by virtue of any written or oral expression relating to such a transaction by any such party or any of its shareholders, directors, officers, employees, representatives, advisor or agents or by virtue of this non-binding indication of interest. In addition, W. R. Berkley may, now or in the future, consider transactions or business ideas similar to the transactions contemplated by this non-binding indication of interest, either alone or with other third parties, and nothing in this non-binding indication of interest shall prevent W. R. Berkley from pursuing any such transaction or business ideas or pursuing businesses similar to or related to the transaction contemplated hereby.

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Advisors: Any questions with respect to this Proposal should be directed to:

Richard Baio

Vice President – Treasurer

W. R. Berkley Corporation

475 Steamboat Road

Greenwich, Connecticut 06830

(203) 629-3000

rbaio@wrberkley.com

In addition, our legal advisors may be contacted as follows:

Mark Cognetti

Willkie Farr & Gallagher LLP

787 Seventh Avenue

New York, New York 10019

212-728-8968

mcognetti@willkie.com

*  *  *  *  *

W. R. Berkley is extremely excited about the prospects of this Proposal and looks forward to working with the Company to move this transaction to a quick signing and closing. Please call us with any questions regarding this Proposal.

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