The eight drug offenders’ whose prison terms Obama commuted were sentenced under a system that treated convictions for crack cocaine offenses harsher than powder cocaine. Yasuyoshi Chiba/AFP/Getty Images

President Barack Obama on Thursday commuted the sentences of eight people he said were serving unduly harsh sentences for drug crimes in the most expansive use yet of his presidential power to free inmates.

Obama made the commutations because the prisoners were sentenced under a system that treated convictions for crack cocaine offenses harsher than powder cocaine. Obama also pardoned 13 others for various crimes.

The president signed the Fair Sentencing Act in 2010 to reduce penalties for crack-cocaine offenses in order to reduce the disparity with powder-cocaine penalties. But the act only addressed new cases, not old ones.

Obama said those whose sentences he commuted have served at least 15 years in prison, many under mandatory minimums that required judges to impose the long sentences even if they didn't think it fit the crime.

"If they had been sentenced under the current law, many of them would have already served their time and paid their debt to society," Obama said in a written statement. "Instead, because of a disparity in the law that is now recognized as unjust, they remain in prison, separated from their families and their communities, at a cost of millions of taxpayer dollars each year."

In the previous five years of his presidency, Obama had only commuted one drug sentence and pardoned 39 people. A pardon forgives a crime and wipes out the conviction, typically after the sentence has been served. A commutation leaves the conviction but ends the punishment.

Groups that advocate for prisoners have criticized Obama for being stingy with his power - George W. Bush granted 189 petitions for pardon and 11 for clemency, while Bill Clinton granted 396 for pardon and 61 for clemency. White House officials say Obama had only approved a single clemency petition among more than 8,000 received because it's the only one that had been given a positive recommendation by the Justice Department.

The old sentencing guidelines subjected tens of thousands of black people to long prison terms for crack cocaine convictions while giving far more lenient sentences to those caught with powder who were more likely to be white. It was enacted in 1986 when crack cocaine use was rampant and considered a particularly violent drug. Under that law, a person convicted of possessing five grams of crack cocaine got the same mandatory prison term as someone with 500 grams - 100 times - of powder cocaine. The Fair Sentencing Act reduced the ratio to about 18-1 and eliminated a five-year mandatory minimum for first-time possession of crack.