A stampede of federal prisoners is about to go home early, thanks to a change in federal sentencing guidelines that's cut short their penalties for drug crimes.

The exodus is expected to include about 6,000 people – nearly 3 percent of the overall federal prison population – who will leave penitentiaries between Oct. 30 and Nov. 2. It's being reported as the largest one-time release in U.S. history.

About a third of those released will be foreign citizens who then will be deported, The Washington Post reported Tuesday in an article that attracted public attention to the large number of pending releases.

The sudden interest left some reform advocates scratching their heads. “One thing that surprised me is that it’s not news,” says Michael Collins, policy manager at the Drug Policy Alliance.

“Obviously the time is now upon us, but it’s not like yesterday [President Barack] Obama announced 6,000 prisoners would get out of jail,” he says. “This isn’t a case where 6,000 people randomly are going to get out of prison.”

Indeed, the gush of releases is the result of thousands of deliberative case reviews following the U.S. Sentencing Commission’s decision last year to retroactively apply reduced sentencing guidelines for a slew of drug crimes.

The Sentencing Commission, an independent agency of the judicial branch, voted in July 2014 to retroactively apply a two-level sentencing reduction to many drug convicts, who then could petition judges to shorten their terms. Congress did not block the reform, which takes effect Nov. 1.

An August report by the Sentencing Commission said that more than 13,000 people received sentence reductions from judges, who review applications on a case-by-case basis. More than 4,600 requests were denied for various reasons, including the prisoner's criminal history or the application of mandatory minimum penalties.

On average, prisoners who won reductions under the change received 23 months off their sentence. The cost of imprisoning a federal inmate for one year was more than $30,000 in fiscal year 2014, meaning the government may save close to $1 billion from the retroactive reductions.

It’s unclear what the demographic makeup of the first 6,000 or so about-to-be-released drug convicts will be, but the broader pool of people who received sentencing reductions is diverse in terms of the race of the offender and the drug crime involved.

The larger pool is about 93 percent male and about 38.5 percent Hispanic. Whites make up 24 percent and blacks 34 percent of those who received reductions. Foreign citizens comprise about 24 percent.

Methamphetamine crimes account for nearly a third of the convictions, followed closely in number by powder cocaine charges. Crack cocaine was involved in about a fifth of cases. Less than 10 percent of beneficiaries were imprisoned for marijuana or heroin crimes.

U.S. Sentencing Commission

Emily Pierce, a spokeswoman for the Justice Department, says the 6,000 figure being reported “is currently an estimate because we could have more orders from judges that come in during the next few weeks,” though the department isn’t expecting many more.

A spokesman for the Sentencing Commission was unable to confirm the number of people who will be released on or around Nov. 1, saying that the Bureau of Prisons, a branch of the Justice Department, is responsible for keeping tabs on release dates.

The mass release comes as a bipartisan alliance on Capitol Hill appears to be gaining traction toward passing legislation that would lower some drug penalties and do away with harsh mandatory minimum sentences for drug crimes, which the commission would not be able to do.

A new sentencing reform bill was introduced in the Senate last week by a far-flung coalition including Sens. Patrick Leahy, D-Vt., Lindsey Graham, R-S.C., Cory Booker, D-N.J., John Cornyn, R-Texas, Sheldon Whitehouse, D-R.I., Mike Lee, R-Utah and Chuck Schumer, D-N.Y

Obama, meanwhile, has used his presidential powers to free some drug convicts, though he generally chooses to shorten terms rather than give pardons. In July, he announced he was commuting the sentences of 46 people convicted of non-violent drug crimes. Those commutations take effect Nov. 10.

Momentum toward reform has met some resistance. After Obama's action in July, the president of the National Association of Assistant U.S. Attorneys campaigned against reform in a speech at the National Press Club, saying it would be a "huge mistake" to pass sentencing reform legislation. He suggested building more prisons.

Collins, however, says he's hopeful Obama soon will have a reform bill on his desk.