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Attorney General Merrick Garland instructed federal prosecutors Friday to end sentencing disparities between offenders convicted of crimes involving crack and powder cocaine, a decadeslong policy that has led to disproportionate prison sentences for Black Americans compared to Whites.

Garland’s memo states that the disparity is “simply not supported by science,” as there are “no significant pharmacological differences between the drugs.”

Currently, a five-year mandatory sentence is triggered for possession of 28 grams of crack cocaine, while the same mandatory sentence for possession of powder cocaine requires 500 grams – a nearly 18 to 1 disparity.

In practice, the disparities have led to a disproportionate share of people of color receiving higher conviction rates than similarly situated White defendants, according to the US Sentencing Commission.

’10 years is enough’: Advocates and bipartisan lawmakers push for sentencing reform for drug-related offenders

The new policy, which becomes effective in 30 days, changes the requirements needed to trigger mandatory minimums for crack-related convictions. Garland’s memo tells prosecutors to take into consideration factors such as whether a defendant is accused of violence, whether they are a member of an organized gang or cartel and whether they had a significant managerial role in a drug trafficking operation.

Civil rights organizations have pushed to end crack-related sentencing disparities for decades, and attorneys general across previous administrations have promised to ease the significant disparities without much success.

“The sentencing disparity between crack and powder cocaine has just one single purpose: to put Black Americans in jail. That’s it,” NAACP President Derrick Johnson said in a statement. “There is no scientific justification for prosecuting and sentencing crack and powder offenses differently. It does not make our communities safer and has simply been used as a tool to lock our community up in jail in the failed War on Drugs.”

Johnson added that “today’s announcement is another step toward restoring faith in the criminal justice system for Black Americans.”

Garland’s memo also reiterated the department’s support for the bipartisan Equal Act, which would end the crack-to-powder sentencing disparity in federal law.

CNN’s Holmes Lybrand contributed to this report.