They could see the meltdown coming.

Freelance financial watchdogs who examined the paperwork on sub-prime home loans being sold to Wall Street had an inside view of the boom in easy-money lending this decade. The reviewers say they raised plenty of red flags about flaws so serious that mortgages should have been rejected outright -- such as borrowers' incomes that seemed inflated or documents that looked fake -- but the problems were glossed over, ignored or stricken from reports.

The loan reviewers' role was just one of several safeguards -- including home appraisals, lending standards and ratings on mortgage-backed bonds -- that were built into the country's complex mortgage-financing system. But in the chain of brokers, lenders and investment banks that transformed mortgages into securities sold worldwide, no one seemed to care about loans that looked bad from the start. Yet profit abounded -- until defaults spawned hundreds of billions of dollars in losses on mortgage-backed securities.

"The investors were paying us big money to filter this business," said Cesar P. Valenz, one of the loan checkers. "It's like with water. If you don't filter it, it's dangerous. And it didn't get filtered."

As foreclosures mount and home prices skid, the loan review function, known as due diligence, is gaining attention. The FBI is conducting more than a dozen probes into whether companies along the financing chain concealed problems with mortgages. And a presidential working group has blamed the sub-prime debacle in part on a lack of diligence by investment banks, rating firms and mortgage-bond buyers.

"Although market participants had economic incentives to conduct due diligence," the group said in a policy statement, "the steps they took were insufficient." To prevent mortgage crises, the group recommended increased disclosure of "the level and scope of due diligence performed" on home loans underlying the securities.

At the height of the sub-prime era, such disclosure wouldn't have been pretty, the freelance loan checkers say.

In interviews with The Times, eight experienced loan reviewers said that as marginal lending increased, quantity took precedence over quality. Squads of 10 to 15 veteran loan checkers gave way, they said, to packs of 40 to 50 mostly novice reviewers posted at or near sub-prime factories such as now-defunct Orange County lenders New Century Financial Corp. and Ameriquest Mortgage Co.

Executives at the two main firms that hired the freelancers -- Shelton, Conn.-based Clayton Holdings Inc. and San Francisco-based Bohan Group -- say the reviewers weren't there to find every potential problem with a sub-prime loan. Rather, the executives say, the job was to perform specific tests to help buyers determine how much to pay for a pool of loans. In some cases, the investors wanted only minimal testing, said Frank P. Filipps, Clayton's chairman and CEO.

"The client really drives the process," Filipps said.

Sub-prime mortgages skyrocketed in popularity -- with the volume of sub-prime-backed securities soaring from $13 billion in 1995 to $594 billion in 2005 and $521 billion in 2006 -- and business exploded for Clayton and Bohan. At the peak, Clayton had about 900 loan-review contractors working for it at any given time, and privately held Bohan had about 350. At publicly held Clayton, revenue rose from $19 million in 2000 to $239.2 million in 2006.

As time passed, Clayton and Bohan executives said, Wall Street firms and their investor customers accepted increasing levels of default and fraud in sub-prime loans as they grew to trust software designed to offset those risks by charging higher interest rates, extra fees and penalties for paying off mortgages early.

As Wall Street grew more comfortable, it demanded less of the review process. Early in the decade, a securities firm might have asked Clayton to review 25% to 40% of the sub-prime loans in a pool, compared with typically 10% in 2006, although the requirements varied, Filipps said.

By contrast, loan buyers who kept the mortgages as an investment instead of packaging them into securities would have 50% to 100% of the loans examined, Bohan President Mark Hughes said.

But the freelancers interviewed by The Times never got the memo that their reviews were supposed to be nice and easy. Flying from city to city and typically paid $30 to $40 an hour, with expenses covered, the reviewers say they worked conscientiously to assure the investment banks and mortgage-bond investors that no surprises lay in the files.

Loan reviewer Jana Lujan recalled showing a file to a supervisor in 2004, during a check of sub-prime mortgages made by a Brea bank that regulators later cited for unsound lending. A title report showed a tax lien on the property.

"I said we needed evidence it had been paid off and released," to ensure against foreclosure, Lujan said. "And he said: 'Just go ahead. Assume it's being taken care of.' "

Loan-buyer representatives who were on site during the reviews also showed little interest in the details, Lujan said.

Lujan said one Clayton supervisor would throw away documents that appeared to have been altered fraudulently. The lack of a document in the file meant the loan had to be sold at a slight discount, she said, but it still could be sold.

Lujan, Valenz and one other loan checker said supervisors at Clayton and Bohan also would change the way fees were described so that mortgages would not be red-flagged as potentially predatory under U.S. law, which would render them unsalable and force the sellers to take them back.