At least 134 prison inmates are officially listed as on the loose across the U.S., a survey of 50 states by the Associated Press shows. Most of these escapees have been on the run for years.

For the survey, the AP gathered data of current escapees from state prisons. Some correctional facilities, however, provided data for recent decades, the agency said.

The review comes after the June 6 New York prison escape when convicted murderers Richard Matt and David Sweat broke out from the Clinton Correctional Facility, a maximum security prison. The two used power tools and a series of tunnels to escape, allegedly with the cooperation of a prison worker.

However, according to the study, instances of inmates successfully fleeing from prisons are rare. At least 24 states said that no prisoners escaped from their correctional facilities.

“The U.S. Bureau of Justice Statistics reported about 2,000 state and federal inmates escaped or went off without leave in 2013. But the figure doesn't indicate how many were caught and does not distinguish between breaking out of prison and walking away from work release or other unfenced settings,” the AP reported.

The survey found that as the years pass, it becomes difficult for the corrections officials to track the escaped prisoners, but the search for the runway inmates continues. Oklahoma corrections chief James Saffle, who has been tracing escaped convicts for 11 years, told AP: "You don't forget about them."

"Sometimes, some little action they take will trigger something," he said.

According to the AP, state prison officials re-examine escape cases, check on the associates of the escaped prisoners, and sift through fingerprint databases and death certificates, to get the latest information. However, they mostly wait for a tipoff about the escaped inmates, the AP reported.

Chuck Jordan, president of the National Association of Fugitive Recovery Agents, told AP that often the escaped prisoners are caught quickly, but the search for the fugitives usually becomes difficult after six months. Moreover, following up on decades-old cases becomes tough due to old paper records, he added.

Michigan prison officials reportedly review the cases of the fugitive inmates every six months. Lt. Craig Cvetan, spokesman for the Ohio State Highway Patrol, told AP that the department assesses criminal records, death certificates and social media every year for clues on cold cases. "You don't say, 'Well, if we haven't found the person by five years, we're not going to do anything else with it,'" he said.