Attorney General Eric Holder directed federal prosecutors on Thursday to use DNA evidence to the greatest extent possible, reversing a policy of the Bush administration.

In guidance to U.S. attorneys' offices around the country, the attorney general said collection of DNA samples from people who are arrested must be a priority of the Justice Department.

"DNA evidence has helped ensure that justice is done," Holder said in one of two memos to prosecutors. He said that it "should be used whenever possible."

Holder says that starting now, department policy will no longer require defendants who are pleading guilty to waive their rights to DNA testing. The attorney general says that from now on, prosecutors may obtain waivers to DNA testing only under exceptional circumstances.

The attorney general says DNA evidence is one of the most powerful tools available to the criminal justice system.

Senate Judiciary Committee chairman Patrick Leahy, D-Vt., commended Holder "for reversing the Bush administration policy of demanding that defendants waive their right to DNA testing as a requirement of certain plea negotiations."

The Innocence Protection Act in 2004, which Leahy sponsored, allows convicted individuals access to DNA testing if it could produce evidence raising a reasonable probability that the individual did not commit the offense.

The 2004 law applies to evidence that was not previously tested and generally requires it to be done within 36 months of conviction.

"I authored the Innocence Protection Act to ensure that the guilty are punished and the innocent are exonerated," Leahy said in a statement. "The attorney general's decision restores proper prosecutorial discretion and furthers my intent in passing that landmark law."