Two crew members died and two others are missing after a police helicopter crashed into a reservoir in a northern suburb of Beijing Wednesday, local police said.

Altogether five crew members were aboard the helicopter that was returning from a search and rescue drill to the mountainous region of Miyun County, local police announced in a statement.

A crew member was pulled out alive of the water at about 10 a.m., and by 10 p.m. rescuers had retrieved the bodies of two crew members.

Currently, rescuers are still searching for the two other crew members, it said.

As the Miyun reservoir is a major source of drinking water for Beijing residents, local water and environmental authorities said they have been keeping a close watch over the water quality since the crash occurred.

Monitoring results show that no oil contamination was found on the surface of the reservoir, and all indexes of the water quality are at normal levels.

The Beijing Municipal Water Authority has launched an emergency plan to ensure the safety of the reservoir water.

Local police are investigating the cause of the crash.