After the closing bell last night, Thornburg Mortgage, Inc. (NYSE: TMA) managed to report a second-quarter profit, but the firm warned investors that it's in jeopardy of collapse as margin calls continue to roll in. Thornburg said that it covered $219 million of demands for collateral on August 21, and may face another $25.9 million of margin calls. Plus, uncertainty still remains about the outcome of an exchange offer that was meant to pull the New Mexico-based mortgage lender back from the brink of bankruptcy.

The jumbo-loan specialist said it swung to a second-quarter profit of $412.3 million, or 84 cents per share, after swallowing a first-quarter loss of $3.31 billion. During the recently concluded quarter, Thornburg wrote down $209.6 million in mortgage losses, which was offset by a $536.9-million gain from the declining value of a liability. Adjusted income for the period was $22.7 million.

Under the terms of a deal with MatlinPatterson Global Advisers, Thornburg agreed in March to conduct an exchange offer for some preferred stock. The offer expires on September 3, and holders of two-thirds of each of four classes of preferred stock must participate. The company warned that uncertainty about the outcome of the exchange offer, combined with the still-shaky market conditions, "raise substantial doubt about the company's ability to continue as a going concern for the foreseeable future."