For the first time since the year of the Cheshire murders, the number of inmates in Connecticut’s prisons increased in 2013 and state officials are not sure why.

That’s according to statistics compiled monthly by the Office of Policy and Management. A report from the agency’s Criminal Justice Police and Planning Division released this week expects an average of 16,626 inmates this month when they had forecasted having only 16,010.

“There is still no clear explanation about why there are 600 more prisoners incarcerated today than were projected. We also do not know the exact circumstances that kept sentenced offenders in prison longer than they might have been held in the past,” the report read.

The confusion stems from the fact that crimes rates have been in decline and the number of people being sentenced to prison dropped in 2013. Admissions were down 4.3 percent from the previous year. At the same time, fewer inmates were being released in 2013. Discharges dropped 12.2 percent from 2012.

As a result, the prison population expanded by 1.5 percent between Jan. 1, 2013 and Jan. 1, 2014.

The last year in which Connecticut’s prison population grew was 2007, the same year as the infamous Cheshire home invasion murders. During that incident, Jennifer Hawke-Petit and her two daughters were murdered in their home by two men who had been on parole. That year, Gov. M. Jodi Rell stopped parole of all violent offenders and the state’s prison population swelled by 2.8 percent.

Michael Lawlor, the OPM undersecretary who heads the Criminal Justice Police and Planning Division, said he is reasonably confident that the population expansion was related in part to the recent implementation of offender risk assessment policies enacted as a result of the Cheshire case. He said those policies have resulted in a reduction of the number of inmates that were released on parole for a period in the spring of 2013.

Lawlor said the prison population has since returned to its typical rhythm in which it expands and contracts during different seasons of any given year.

“It’s really just a flow of business problem, changing how you handle these cases,” he said. “We’re making sure we do identify the high-risk, violent guys and making sure they are dealt with differently.”

Lawlor said that crime rates continue to decrease and other current trends remain on track, and he expects the state will see a gradual reduction in its prison population in the coming year. He said his agency is also seeking better analytical tools to track and manage the state’s complex prison system.

According to the report, last year’s population increase was mitigated somewhat through the state’s expanded use of a home confinement program called Transitional Placement. The program moves offenders who are doing well in a halfway house back to their homes if the Correction Department considers their home to be an appropriately supervised setting.

“As the prison population began to swell over expected levels during the spring of 2013, the DOC began to investigate the options available for easing population pressures in the prisons. Since little could be done to move a large number of parole cases forward in the short term, Transitional Placement was identified as the means of moving many low-risk offenders out of halfway house beds, thus freeing up those fixed slots for appropriate offenders who were in prison,” the report read.

The program is approved at the discretion of the DOC commissioner. The number of inmates using the program has grown significantly in the past two years. In January 2012, there were six inmates in Transitional Placement compared to 97 this month.