Capmark Financial Group Inc., one of the nation's largest commercial-real-estate lenders, filed for bankruptcy protection in Delaware.

The much-expected move underscores the deep problems in the business-property market. After suffering from the collapse in residential mortgages, U.S. banks face steep losses from commercial real-estate loans. Capmark has originated more than $10 billion in commercial real-estate loans, according to Moody's Investors Service.

It also represents a blow to the company's private-equity owners. In 2006, a group led by KKR & Co., Goldman Sachs Capital Partners and Five Mile Capital Partners acquired the lender GMAC LLC's commercial-real estate business and renamed it Capmark. As of March 31, the investor group owned about 75% of the company, with GMAC and its employees owning the balance.

The Horsham, Pa., company recently reported a $1.6 billion second-quarter loss and warned it might be forced to seek Chapter 11 bankruptcy protection. KKR has already written down its investment in Capmark to zero.

Capmark recently entered an agreement to sell its North American servicing and mortgage-banking operations to a new company owned by Warren Buffet's Berkshire Hathaway and Leucadia National Corp. for as much as $490 million. Under the deal's terms, the sale could occur while Capmark is in bankruptcy, but would require a bigger cash payment.