A report by the department's Bureau of Justice Statistics, released Thursday, found that 6.98 million adults were under some kind of supervision by federal, state or local corrections officers at the end of last year. That represents about 2.9 percent of adults in the U.S.
Last year, the number of people in jail, in prison or on parole or probation decreased by 98,900, or 1.4 percent, the third straight year of decline.
All told, about 7 in 10 people in the corrections system were on probation or parole, with the remainder locked up.
James Alan Fox, a professor of criminology, law and public policy at Northeastern University, said the reduction reflects the cumulative impact of lower crime rates.
"There is a lag between crime drops and correctional drops because of the length of sentences being served," he said. "It is likely that the correctional population will continue to decline as releases outpace admissions."
The overall decrease was fueled mostly by a drop in the number of people on probation, which fell below 4 million for the first time since 2002. The number of people incarcerated also dropped slightly.
One area that saw a small increase was the number of people on parole. Although the number of people who entered parole decreased last year, that was more than offset by fewer people leaving parole.
Fox said the overall downturn also reflected tight budgets and limited space, which have forced authorities to release certain offenders early.
"This can be seen in the increasing numbers of parolees contrasting with the declining numbers of prisoners," he said.