The Pentagon paid hundreds of billions of dollars to defense contractors engaged in criminal or civil fraud -- in some cases paying the companies after they were convicted, according to a new Defense Department report.

At least 91 contractors holding contracts worth $270 billion were the subjects of civil fraud judgments -- and in some cases criminal fraud convictions as well, many of which resulted in fines, suspensions or debarments. Even so, Defense Department contracting officers still assigned $4.9 billion worth of work with these companies after the fraud was uncovered, the report said.

The contractors identified in the report include such blue-chip entities as Boeing, Lockheed Martin, General Dynamics, Pratt & Whitney, IBM and even the Yale medical school.

Under the pressure of two hot wars and a defense budget that has mushroomed to its current $700 billion size, the Pentagon has set up a Procurement Fraud Working Group. But in the six years the task force has been at work, fraud seems to have expanded even as the Pentagon's ability to detect it has increased.

The Pentagon said its own sloppy bookkeeping, missing records and inadequate training of acquisition officials were to blame for the mess. The Pentagon also acknowledged that the data in the report, which covers only 2007 through 2009, are probably incomplete because the Defense Department cannot accurately track individual contractors or how much is actually spent. And the report only tracks those fraud cases of over $1 million.

At least $682 million was paid in error to 30 companies that had been convicted of criminal fraud, according to the report, including $29.6 million to Herley Industries, an electronics manufacturer, and $66 million to AEY Inc., a military surplus and sporting goods firm.

The number of fraudulent contractors identified in the report is relatively small -- some 120 companies out of the 235,000 contractors that work for the Defense Department. But the dollar amounts are significant, a continuing drain on taxpayers and on the military's combat readiness.

Military contracting fraud, which first plagued Gen. George Washington during the Revolutionary War, has long bedeviled the American armed forces. Defense Secretary Robert Gates is only the latest of a long succession of defense chiefs to try to crack down on waste, fraud and abuse in the Pentagon's giant acquisition bureaucracy.

Under the pressure of two hot wars and a defense budget that has mushroomed to its current $700 billion size, the Pentagon has set up a Procurement Fraud Working Group. But in the six years the task force has been at work, fraud seems to have expanded even as the Pentagon's ability to detect it has increased.

With the data in the new report, the Pentagon acknowledged, came "recognition that additional guidance and training . . . is needed'' for its contracting officers.

The report was released by Sen. Bernie Sanders of Vermont, who had demanded a Pentagon accounting of fraud. "The sad truth is that virtually all of the major defense contractors in this country for years have been engaged in systemic fraudulent behavior, while receiving hundreds of billions of dollars of taxpayer money," he said in a statement.

"It is clear that DOD's current approach is not working and we need far more vigorous enforcement to protect taxpayers from massive fraud," Sanders said.