The federal prison population has dropped by nearly 5,000 inmates this year, the first decline in decades, according to the Justice Department.

In a speech Tuesday at New York University’s Brennan Center for Justice, Attorney General Eric H. Holder Jr. highlighted the decline as a breakthrough for criminal-justice reform advocates who have tried to reverse the trend of rising incarceration. He said that in fiscal 2016, the federal prison population is projected to drop by 10,000 inmates, or the equivalent of six federal prisons.

“Statistics have shown — and all of us have seen — that high incarceration rates and longer-than-necessary prison terms have not played a significant role in materially improving public safety, reducing crime or strengthening communities,” Holder said.

Prison reform has been a central theme of the second term of the Obama administration. A year ago, Holder unveiled a prison reform package he called “Smart on Crime” and announced that low-level nonviolent drug offenders with no ties to gangs or large-scale drug organizations would no longer be charged with offenses that impose severe mandatory sentences.

In July, the U.S. Sentencing Commission decided that nearly 50,000 federal drug offenders currently in prison would be eligible for reduced sentences, a plan that was supported by Holder and endorsed by the Justice Department.

The Justice Department will report next week, at the end of fiscal 2014, that the federal prison population will show a decline of 4,800 from the federal inmate population one year ago to about 215,000 inmates. This is the first time that the federal inmate population has fallen, rather than risen, over the course of a fiscal year since 1980, according to a Justice Department official.

“Our new projections anticipate that the number of federal inmates will fall by just over 2,000 in the next 12 months — and by almost 10,000 in the year after,” Holder said. “This is nothing less than historic. To put these numbers in perspective, 10,000 inmates is the rough equivalent of the combined populations of six federal prisons, each filled to capacity.”

Holder said, however, that the projected decreases will not result in any prison closures because the prison system is operating at about 30 percent above capacity. The cost of incarceration in the United States was $80 billion in 2010, according to the Justice Department. While the U.S. population has increased by about one-third since 1980, the federal prison population has grown by about 800 percent.

“But my hope is that we’re witnessing the start of a trend that will only accelerate” as prison reform changes take effect, Holder said.