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Attorney General Eric Holder says that he will soon call on Congress to lower the standard of proof in federal civil rights cases, to allow federal prosecution where local authorities are unable or unwilling to get a conviction.

"There is a better way in which we could have federal involvement in these kinds of matters to allow the federal government to be a better backstop in examining these cases," Holder said in an NBC News interview conducted on Thursday.

On Tuesday, the Justice Department announced that it found insufficient evidence to pursue federal criminal civil rights charges in the 2012 shooting death of a black teenager, Trayvon Martin, in Florida.

In a written statement, government lawyers said their decision in the case was "limited strictly to the department's inability to meet the high legal standard" in the civil rights statutes.

Holder is expected to announce within the next few weeks whether the government will bring civil rights charges stemming from last year's shooting of an unarmed black man, Michael Brown, by a white police officer in Ferguson, Missouri. Officials familiar with the case have said they expect that no federal charges will be filed.

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"We do need to change the law. I do think the standard is too high," Holder said Thursday. "There needs to be a change with regard to the standard of proof."

To bring a federal case, federal prosecutors must prove that a person used excessive force, willfully —meaning on purpose — with the knowledge that it was wrong.

William Yeomans, a former prosecutor in the Justice Department's Civil Rights Division, said proving intent is the biggest challenge in bringing a successful prosecution in such cases.

"It's extremely difficult to establish beyond a reasonable doubt what was in the defendant's mind when he committed the act."

Federal prosecutors brought just such a charge in 1991 against Los Angeles police officers for beating a black driver, Rodney King, after a high-speech chase. But the government had videotape to help prove that case.

The high bar in the law often prevents the federal government from prosecuting. Examples include the New York police shooting deaths of Amadou Diallo, unarmed when he was killed outside his apartment in 1999, and Sean Bell, who died after officers fired 50 shots into his car. No federal charges were filed in either case.

"I'm going to have some specific proposals that we will share with the American people and with Congress," Holder said Thursday.

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