Updated

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 addressed only 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."

Previously, Obama had commuted only one sentence in the five years of his presidency, involving another drug case. He previously had pardoned 39 people. A pardon forgives a crime and wipes out the conviction, while a commutation leaves the conviction but ends the punishment.

In August, Attorney General Eric Holder announced a major shift in federal sentencing policies, targeting long mandatory terms that he said have flooded the nation's prisons with low-level drug offenders and diverted crime-fighting dollars that could be far better spent.

As a first step, Holder has instructed federal prosecutors to stop charging many nonviolent drug defendants with offenses that carry mandatory minimum sentences. His next step will be working with a bipartisan group in Congress to give judges greater discretion in sentencing.

Obama encouraged Congress to act in the new year to bring more fairness to the system.

"Commuting the sentences of these eight Americans is an important step toward restoring fundamental ideals of justice and fairness," Obama said. "But it must not be the last. In the new year, lawmakers should act on the kinds of bipartisan sentencing reform measures already working their way through Congress.  Together, we must ensure that our taxpayer dollars are spent wisely, and that our justice system keeps its basic promise of equal treatment for all."