The prosecutor announced that the helicopter came under fire during take off from Essider, the spokesman for Benghazi Joint Security Room Ibrahim al-Sharaa said.
The perpetrators identify with a group called al-Isnad Alamni, the Libya Herald reported.
The helicopter had taken off quickly because the armed militants had entered the airport, Sharaa said.
Gunman opened fire on the helicopter, which went out to sea, where it likely came down.
Authorities are still searching for wreckage from the aircraft and the bodies of the five men on board, the Herald said.