The federal Bureau of Prisons has released 6,000 prisoners over a four day period, implementing a decision last year to slash the number of incarcerated drug offenders by nearly half. Officials said the nationwide releases from Oct. 30th and Nov. 2nd will be the largest in U.S. history. About 3,400 inmates were already confined in places like halfway houses. Another 1,700 of the inmates are not US citizens and will possibly face deportation.

The 6,000 prisoners are the first wave of early releases. Most of those released will see about two years trimmed from their original prison terms. The changes come as part of an effort to reverse long jail terms for non-violent offenders, who make up about half of the federal prison population.

Last year, in line with a concerted effort by the Obama administration to reduce the number of drug offenders in U.S. prisons, the U.S. Sentencing Commission voted to cut drug sentences by an average of two years, potentially affecting as many as 46,000 of 100,000 cases.

The Obama administration and a bipartisan coalition in Congress have also been working on reforms to lower the country’s incarceration rate, one of the highest in the world. The US has almost a quarter of the world’s prison population – one in every 99 Americans is behind bars.

When the mass release was first proposed, some federal prosecutors and even then-Atty. Gen. Eric H. Holder Jr. were opposed to it. Under a compromise, the sentencing commission agreed to delay the releases until late this year, permitting a more thorough vetting of each case. Congress, where sentencing reform has had bipartisan support, allowed the changes to go into effect.

Officials said the releases are not limited to non-violent offenders, though each release is individually approved by a federal judge, who must make a determination that the person is not a threat to public safety. So far, judges are approving about 75% of requests for release.

In the coming year, an additional 8,550 prisoners will be eligible for release, according to Sentencing Commission spokesman Matt Osterrieder, though he said that not all of them will be approved. The planned releases continue a drive that started in 2013 when US Attorney General Eric Holder dropped mandatory minimum sentences for non-violent drug offenders.

The U.S. Sentencing Commission — an independent agency that sets sentencing policies for federal crimes —reduced the potential punishment for future drug offenders last year and then made that change retroactive. The commission’s action is separate from any effort by President Obama to grant clemency to certain nonviolent drug offenders, an initiative that has resulted in the early release of 89 inmates. Many leading Democrats and Republicans now believe that mandatory prison sentences that led to mass incarceration in the US have not been effective.