WASHINGTON — Attorney General Eric Holder Jr. said Thursday that he has directed federal prosecutors across the country to review and possibly refile charges in ongoing drug cases so that low-level, nonviolent offenders will not face severe mandatory sentences.

The policy change will be applied to suspects in drug cases who have been charged but not yet tried, as well as to individuals who have been convicted but not yet sentenced. The directive does not affect offenders already sentenced or serving time in prison.

As part of a sweeping new policy shift, Holder announced last month that, in future drug cases, low-level, nonviolent suspects would no longer be charged with offenses that impose severe mandatory sentences. The new directive marked an expansion of that effort, ordering U.S. attorneys to apply the policy retroactively.

“I am pleased to announce today that the department has issued new guidance to apply our updated charging policy not only to new matters, but also to pending cases where the defendant was charged before the policy was issued but is still awaiting adjudication of guilt,” Holder said in a speech at the annual conference of the Congressional Black Caucus.

Justice Department spokesman Brian Fallon said department officials do not know how many people will be affected by the new policy. He said Holder has asked for a survey of his 94 U.S. attorneys to determine how many cases will be affected retroactively.

“By reserving the most severe prison terms for serious, high-level or violent drug traffickers or kingpins, we can better enhance public safety,” Holder said in his speech. “We can increase our focus on proven strategies for deterrence and rehabilitation, and we can do so while making our expenditures smarter and more productive.”

The Justice Department’s new policy applies to offenders with no ties to gangs or large-scale drug organizations and those whose offenses did not involve the use of a weapon or violence. Under the guidelines, the offender also cannot have sold drugs to minors or have a significant criminal history.

“Some federal drug statutes that mandate inflexible sentences — regardless of the individual conduct at issue in a particular case — do not serve public safety when they’re applied indiscriminately,” Holder said in his speech.

Holder sent a three-page memorandum explaining the new retroactive guidelines to his prosecutors on Aug. 29. The memo instructs prosecutors not to “disturb the sentence in a case in which the sentence has been imposed.”

In the case of a defendant who was convicted but has not been sentenced, the U.S. attorney has prosecutorial discretion about whether to apply the new guidelines.

The cost of incarceration in the United States was $80 billion in 2010, according to Justice officials. While the U.S. population has increased by about a third since 1980, the federal prison population has grown by 800 percent, and federal prisons are operating at nearly 40 percent over capacity.

Although the United States is home to 5 percent of the world’s population, almost a quarter of the world’s prisoners are in U.S. prisons, according to the Justice Department. There are more than 219,000 federal inmates, and almost half of them are serving time for drug-related crimes.

The new sentencing guidelines are part of a comprehensive prison reform that Holder said he wants to be part of his legacy as attorney general.

Last month, Holder announced that the Justice Department would not challenge laws legalizing marijuana in Colorado and Washington as long as those states maintained strict rules regulating the sale and distribution of the drug.

Supporters of the new state laws had argued that hundreds of millions of dollars had been wasted on a failed war against marijuana that has filled U.S. prisons with low-level, nonviolent offenders.

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