The hedge fund manager Bill Ackman, who correctly predicted shares of MBIA Inc. and Ambac Financial Group Inc. would tumble, said he now is betting against Financial Security Assurance Holdings Ltd.

Financial Security may be insolvent because it sold investment contracts backed by mortgage securities that have declined in value, Mr. Ackman said Wednesday at a conference in New York hosted by the law firm Jones Day. Financial Security, a New York unit of Dexia SA of Brussels, is one of two bond insurers to retain their triple-A credit ratings after rivals were roiled by losses from collateralized debt obligations.

"The market has not woken up to FSA," said Mr. Ackman, who runs the $6 billion Pershing Square Capital Management fund. He said he will make hundreds of millions of dollars if MBIA and Ambac go bankrupt. "FSA is triple-A stable," he said. "Just don't look too close."

Like MBIA, of Armonk, N.Y., and the other bond insurers, Financial Security expanded into guaranteeing and investing in mortgage securities, Mr. Ackman said. Investors have not scrutinized Financial Security's investments closely enough and the ratings companies are misjudging the risks, he said. Mr. Ackman has criticized the insurers for expanding beyond municipal debt into insuring CDOs. He said it may ultimately bankrupt the companies.

Ulrike Pommee, a spokeswoman for Dexia in Brussels, would not discuss Mr. Ackman's remarks. Tom Vogel, a spokesman for Financial Security, did not immediately return a message left on his mobile phone after regular office hours.

Mr. Ackman said he is betting against FSA by buying credit-default swaps that insure the holder against a default on the company's debt. The cost of such swaps almost tripled the past month, according to CMA Datavision of London.

Financial Security offers investment contracts with a return guaranteed by its insurance unit to municipalities and other investors looking to park the proceeds of bond issues. The value of some of the securities backing those contracts has fallen so much that the company's liabilities of $20.4 billion exceed its assets ($16.2 billion), Mr. Ackman said.

Financial Security took share from MBIA and Ambac while their credit ratings were being reviewed, deterring issuers from seeking their guarantees. Fitch Inc. and Standard & Poor's Corp. cut them, to AA from AAA, and Moody's Investors Service Inc. may strip them of their top ratings.

Financial Security and Assured Guaranty Ltd.'s insurance unit were the only bond insurers to keep a top rating and stable outlook from Fitch, Moody's, and S&P. Mr. Ackman said he has not analyzed Assured.

"There's not likely to be a man left standing" in the bond insurance industry, he said. "This thing is over already. The market just doesn't know it yet."

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