The practice of using DNA waivers began several years ago as a response to the Innocence Protection Act of 2004, which allowed federal inmates to seek post-conviction DNA tests to prove their innocence. More than 240 wrongly convicted people have been exonerated by such tests, including 17 on death row.

The waivers are filed only in guilty pleas and bar defendants from ever requesting DNA testing, even if new evidence emerges. Prosecutors who use them, including some of the nation’s most prominent U.S. attorneys, say people who have admitted guilt should not be able to file frivolous petitions for testing. They say the wave of DNA exonerations has little impact in federal court because all those found to be innocent were state prisoners, and the waivers apply only to federal charges. DNA evidence is used far more frequently in state courts.

But DNA experts say that’s about to change because more sophisticated testing will soon bring biological evidence into federal courtrooms for a wider variety of crimes. Defense lawyers who have worked on DNA appeals strongly oppose the waivers, saying that innocent people sometimes plead guilty — mainly to get lighter sentences — and that denying them the ability to prove their innocence violates a fundamental right. One quarter of the 243 people exonerated by DNA had falsely confessed to crimes they didn’t commit, and 16 of them pleaded guilty.

“It’s a mean-spirited policy. Truth, ascertained by science, should trump the finality of a conviction,” said Peter Neufeld, co-director of the New York-based Innocence Project. He said the waivers are effectively “gutting the impact” of the 2004 law because 97 percent of federal convictions result from guilty pleas.

Interviews and documents show that language allowing for DNA waivers was inserted into the law at the behest of Republican senators and that the Bush Justice Department lobbied against the measure even with the waiver provision. Soon after the law passed with bipartisan support, the department sent a secret memo to the nation’s 94 U.S. attorney’s offices urging them to use the waivers, several federal officials familiar with the memo said.

Holder, a former U.S. attorney in the District, has called for expanded DNA testing in federal courts. After inquiries by The Washington Post, his spokesman, Matthew Miller, said Holder “has ordered that the department review its DNA waiver policy.”

“The attorney general believes that DNA testing is a crucial law enforcement tool both in solving crimes and exonerating the innocent,” Miller said, adding that if new evidence arises after conviction, “prosecutors have an obligation to act.”

The waivers run counter to the national movement toward post-conviction DNA testingas the forensic tool has revolutionized criminal justice. Nearly all 50 states have passed laws giving inmates the right to seek testing in state courts, and most allow for petitions after guilty pleas.

Oregon prosecutor Joshua Marquis, who sits on the executive committee of the National District Attorneys Association, said he’s never heard of DNA waivers in state court and that the organization opposes the concept. “I think it’s important to always leave the door open for actual proof of innocence,” he said.

In federal court, the waivers are part of the standard plea agreement filed by prosecutors in the District, Alexandria and Manhattan, which are among the nation’s highest-profile U.S. attorney’s offices. Waivers are used in some or all pleas by at least 16 other offices, including such large ones as Chicago and Los Angeles and such smaller ones as Arkansas and West Virginia. Prosecutors in Maryland rarely use the waivers.

“It saves us a lot of spurious litigation down the pike,” said G.F. Peterman III, acting U.S. attorney in the Middle District of Georgia. “All they have to do is say I’m not guilty, go to trial and they’ve waived nothing. It’s their decision.”

Defense attorneys disagree, saying prosecutors give defendants the choice of signing the waiver or not getting the benefits of a plea agreement, which usually include a lighter sentence.