All five members of the crew aboard a Russian Mi-8 transport helicopter lost their lives when the aircraft crashed in the Khabarovsk region of the Russian Far East, law enforcement officials reported on Wednesday.

The helicopter disappeared from radar on Monday while flying back to its base in Magadan from repairs in the village of Chumikan, with a route that included Okhotsk. Officials noted that it did not send out a distress signal and its emergency locator beacon failed to activate.

Search crews discovered the wreckage three days later on a mountainside located 130 kilometers (81 miles) northeast of Okhotsk, as reported by transportation investigators.

Rescue teams retrieved the bodies of all five individuals on board, which included three crew members and two engineers, according to investigators.

Regional prosecutors announced that they have begun an investigation into the helicopter’s operator, Vzlyot, a company based in Magadan.

A criminal inquiry has been initiated regarding possible breaches of aviation safety regulations.