Millions of U.S. mortgages have been shuttled around the global financial system - sold and resold by firms - without the documents that traditionally prove who legally owns the loans.

Now, as many of these loans have fallen into default and banks have sought to seize homes, judges around the country have increasingly ruled that lenders had no right to foreclose, because they lacked clear title.

These fundamental concerns over ownership extend beyond those that surfaced over the past two weeks amid reports of fraudulent loan documents and corporate "robo-signers."

The court decisions, should they continue to spread, could call into doubt the ownership of mortgages throughout the country, raising urgent challenges for both the real estate market and the wider financial system.

For struggling homeowners trying to avoid foreclosure, it could mean an opportunity to challenge the banks they argue have been unhelpful at best and deceptive at worst. But it also threatens to leave them in prolonged limbo, stuck in homes they still can't afford and waiting for the foreclosure process to begin anew.

For big banks, "there's a possible nightmare scenario here that no foreclosure is valid," said Nancy Bush, a banking analyst from NAB Research. If millions of foreclosures past and present were invalidated because of the way the hurried securitization process muddied the chain of ownership, banks could face lawsuits from homeowners and from investors who bought stakes in the mortgage securities - an expensive and potentially crippling proposition.

For the fragile housing market, already clogged with foreclosure cases, it could mean gridlock and confusion for years. And there is concern in Washington that if the real estate market and financial institutions suffer harm, it could force the government to step in again. Attorney General Eric H. Holder Jr. said Wednesday he is looking into the allegations of improper foreclosures, and Sen. Christopher J. Dodd (D-Conn.), chairman of the Senate banking committee, said he plans to hold hearings on the issue.

At the core of the fights over the legal standing of banks in foreclosure cases is Mortgage Electronic Registration Systems, based in Reston.

The company, known as MERS, was created more than a decade ago by the mortgage industry, including mortgage giants Fannie Mae and Freddie Mac, GMAC, and the Mortgage Bankers Association.

MERS allowed big financial firms to trade mortgages at lightning speed while largely bypassing local property laws throughout the country that required new forms and filing fees each time a loan changed hands, lawyers say.

The idea behind it was to build a centralized registry to track loans electronically as they were traded by big financial firms. Without this system, the business of creating massive securities made of thousands of mortgages would likely have never taken off. The company's role caused few objections until millions of homes began to fall into foreclosure.

In recent years, the company has faced numerous court challenges, including separate class-action lawsuits in California and Nevada - the epicenter of the foreclosure crisis. Lawyers in other states have also challenged the company's legal standing in court.