Some of the nation's largest mortgage companies used a single document processor who said he signed off on foreclosures without having read the paperwork - an admission that may open the door for homeowners across the country to challenge foreclosure proceedings.

The legal predicament compelled Ally Financial, the nation's fourth-largest home lender, to halt evictions of homeowners in 23 states this week. Now it appears hundreds of other companies, including mortgage giants Fannie Mae and Freddie Mac, may also be affected because they use Ally to service their loans.

As head of Ally's foreclosure document processing team, 41-year-old Jeffrey Stephan was required to review cases to make sure the proceedings were legally justified and the information was accurate. He was also required to sign the documents in the presence of a notary.

In a sworn deposition, he testified that he did neither.

The reason may be the sheer volume of the documents he had to hand-sign: 10,000 a month. Stephan had been at that job for five years.

How the nation's foreclosure system became reliant on the tedious work of a few corporate bureaucrats is still a matter that mortgage lenders are trying to answer. While the lenders may have had legitimate cause to foreclose, the mishandling of the paperwork has given homeowners ammunition in their fight against foreclosure and has drawn the attention of state law enforcement officials.

Ally spokesman James Olecki called the problem with the documents "an important but technical defect." He said the papers were "factually accurate" but conceded that "corrective action" may have to be taken in some cases and that others may "require court intervention."

Olecki said the company services loans "from hundreds of different lenders," but he declined to provide names.

Spokesmen for Fannie and Freddie confirmed Tuesday after inquiries from The Washington Post that they use Ally, formerly called GMAC, to oversee some mortgages. The companies have launched internal reviews to assess the scope of any potential issues.

Ally, Fannie and Freddie - all troubled mortgage companies that received extraordinary bailouts by the federal government during the financial crisis - declined to say how many loans might be affected. The Treasury Department, which owns a majority stake in Ally and seized Fannie and Freddie in 2008, also declined to comment.

Fannie and Freddie, created by Congress to finance mortgages and encourage homeownership, have in recent years been repossessing houses at record numbers. Fannie alone reported recently that 450,000 of its single-family loans were seriously delinquent or in the foreclosure process as of June 30. That's nearly 5 percent of the loans it guarantees.

Lawyers defending homeowners have accused some of the nation's largest lenders of foreclosing on families without verifying all of the information in a case, but it has been hard for them to stop foreclosure proceedings.