Michael Winter and Kim Hjelmgaard

USA TODAY

LONDON — British investigators Wednesday were trying to establish what caused a U.S. Air Force helicopter to crash in a bird sanctuary during a "low-level" training mission along the eastern coast of England in which four crewmembers died.

The HH-60G Pave Hawk, an updated version of the U.S. Army Black Hawk, went down in the Cley Marshes nature reserve on Tuesday evening, in Norfolk.

The helicopter was attached to the 48th Fighter Wing at the Royal Air Force base at Lakenheath, in Suffolk, which is also home to the 56th Rescue Squadron, the U.S. Air Force reported.

The USAF said in a brief statement that the crash occurred about 6 p.m. (1 p.m. ET), though Norfolk police reported it happened about 7 p.m. The local fire department said the first rescue unit arrived at 7:53 p.m.

The area remains sealed off on Wednesday and the probe is expected to take a number of days because the helicopter was carrying live ammunition. No injuries in the immediate area were reported.

Late Tuesday a Pentagon official confirmed that all four crewmembers died in the crash, the Norfolk Eastern Daily Pressreported. The names of the crewmembers have not been released.

Initial reports indicated the aircraft had crashed into the North Sea, but the the Royal National Lifeboat Institution later confirmed it came down over land.

A local official told the BBC the helicopter had crashed in the bird sanctuary, which is frequented by geese and other waterfowl migrating from Greenland, Iceland and North America. Residents speculated the aircraft might have hit a bird or birds over the 400 acres of the protected marshes, which are owned and managed by the Norfolk Wildlife Trust.

The preserve, England's first, has been closed because of recent coastal flooding.

One resident told the BBC he took video of two military aircraft flying "extremely low" about two hours before the crash. Another said he heard the impact and thought the aircraft had hit the beach.

The HH-60G Pave Hawk is primarily used for search and rescue and to remove combat forces from hostile area. It's also used for medical evacuation, disaster response and humanitarian assistance. Pave refers to the aircraft's electronics systems.

RAF Lakenheath is the largest U.S. Air Force-operated base in England and the only F-15 fighter wing of U.S. Air Forces in Europe.

Michael Winter reported from San Francisco.