President Obama signs the Fair Sentencing Act in the Oval Office, Aug. 3, 2010.

The U.S. Congress passed the Fair Sentencing Act in August 2010, reducing the 100-to-1 disparity between minimum sentences for crack and powder cocaine to 18 to 1.

The U.S. Sentencing Commission voted this summer to make the reduced crack penalties retroactive, which means that more than 12,000 current inmates are eligible for reduced sentences.

Before Congress acted, the old guidelines meant someone who possessed just 5 grams of crack cocaine (roughly the weight of a nickel) would receive a mandatory minimum sentence of five years. But someone would have to have 500 grams of powdered cocaine (just over a pound) to receive a similar sentence.

“This really has been one of the great stains on our federal criminal justice system for 20 years or more,” said Michael Nachmanoff, the federal public defender for the Eastern District of Virginia. “This disparity between the punishment for crack cocaine and powder was really unjustified.”

Nachmanoff’s district is believed to have the largest number of people in the country — between 800 and 900 — who might be released early due to the new guidelines. He said 75 of his clients were expected to be released on the first day of retroactivity.

“A lot of people have been sitting in jail for a long time, not because they didn’t commit crimes, but because the punishment they faced was too harsh and unjustified compared to other people who had committed similar crimes in similar ways,” Nachmanoff told CNN. He said reduced sentences will not be automatic. Judges will review the cases and determine whether an early release of an inmate represents a danger to the community.

The retroactivity took effect Nov. 1. An estimated 1,800 inmates became eligible for release immediately because they had already served enough time, and prosecutors did not object to their release.