The number of people incarcerated in the United States fell 10 percent between 2010 and 2017, according to Justice Department statistics released Thursday.

While incarceration rates had been on the rise for decades, the number of prisoners compared to the U.S. population reached a 20-year low in 2017, according to the statistics. The incarceration rate for state and federal prisoners sentenced to more than a year also dropped 13 percent, from 506 prisoners per 100,000 residents in 2007 to 440 prisoners per 100,000 in 2017, according to the Bureau of Justice Statistics. Incarceration in jails fell 12 percent during the same decade, from 259 to 229 jail inmates per 100,000 residents, but stayed flat between 2016 and 2017.

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The prison population was 1.5 million at the end of 2017, compared to 745,000 jail inmates at midyear 2017, according to the Bureau. Between the end of 2016 and the end of 2017, the number of prisoners under federal jurisdiction declined by 6,100, or 3 percent, while the number of prisoners under state jurisdiction fell 1 percent, or 12,600.

The report also found that noncitizens comprise about the same percentage of the prison population, at 7.6 percent, as they do of the total U.S. population.

Criminologist James Alan Fox of Northeastern University told NBC News the drop could largely be attributed to steep declines in the crime rate, noting that violent crime fell 19 percent in the decade covered by the statistics.

Marc Mauer of the Sentencing Project said the decline also had to do with alterations to sentencing policies for drug offenses as well as states taking steps to reduce overcrowding in correctional facilities. While a step in the right direction, he said, the statistics "still fall far short of what is necessary to end mass incarceration anytime soon."