BELGRADE, Serbia — Police have detained five prison guards who were on duty when two convicted murderers escaped from their cells at a high-security lockup in Belgrade, including one who had taken part in the assassination of a Serbian prime minister, officials said Wednesday.

Both men left their cells by using saws to cut the bars, overpowered a guard and jumped out a window into the prison yard on Tuesday, but were caught there by special police.

Police are investigating whether the five guards at the District Prison should be blamed for the attempted escape by Sretko Kalinic and Zeljko Milovanovic and whether the two killers — both serving long terms — had help from outside the lockup, said Interior Minister Ivica Dacic.

"We now must determine whether this was an organized attempt or just an omission" by the prison's security, Dacic said. Authorities released footage from a prison camera showing the two inmates exiting their cells into a prison hall, putting on jackets and hurrying off.

Kalinic is serving a 30-year sentence for his role in the assassination of Serbia's reformist Prime Minister Zoran Djindjic, who was gunned down in 2003 in front of the government headquarters by a group of paramilitaries and criminals. He did not pull the trigger, but was a member of a notorious criminal gang that planned and carried out the killing.

Kalinic, known in Serbia's underground as "the beast" for his brutality, also was sentenced to 40 additional years in prison for other crimes, including executions and robberies.

Milovanovic was sentenced to 40 years in jail in neighboring Croatia for killing a prominent journalist there, Ivo Pukanic, and his associate in 2008. The inmate also is being tried in Serbia for other crimes.

A Serbian newspaper alleged Wednesday that the attempted prison escape had the help of outsiders.

Quoting unidentified investigators, the Blic daily alleged that reputed mafia boss Sreten Jocic provided such assistance. He is currently being tried for allegedly ordering the killing of Pukanic.

Blic also said police are searching buildings around the prison in residential Belgrade to determine if anyone was waiting there for Kalinic and Milovanovic if they managed to escape.

Milan Obradovic, who is in charge of Serbian prisons, said it must be determined how the two inmates obtained and hid the saws in their cells. It wasn't surprising that two inmates serving long sentences wanted to escape, "but it is our job to prevent that," he said.