U.S. military investigators have begun examining the wreckage of a U.S. Air Force Sikorsky HH-60G Pave Hawk helicopter that crashed into a marsh in Norfolk, U.K., killing four airmen.

The accident happened during a nighttime, low-flying training exercise on Jan. 7 when one of a pair of HH-60s, based with the 48th Fighter Wing at RAF Lakenheath, Suffolk, came down on marshland in a nature reserve near the village of Cley-next-the-Sea on the North Norfolk coastline. The second aircraft landed nearby to assist in recovery and remains at the scene of the crash. No one on the ground was injured.

Emergency services cordoned off a large area citing concerns over the live ammunition that was being carried on the aircraft. Officials are examining the wreckage, which is spread over a wide area. Investigators already have removed some wreckage that was vulnerable to being washed away by high tides.

Chief Superintendent Bob Scully of Norfolk Constabulary said: “The crash site is about the size of a football [soccer] pitch, with difficult terrain, which makes this a challenging and lengthy process.”

Scully said police were examining the site on behalf of a coroner, who under English law is in charge of investigating the deaths. Control of the area later will be handed to air investigators from the British and U.S. military.

As well as police, officials from the U.K. defense ministry, U.K. Air Accident Investigation Branch and U.S. Air Force were assessing the site.

The type is a familiar sight in the skies over Norfolk, as the aircraft regularly use the former RAF Sculthorpe airbase as well as U.K. Royal Air Force weapons ranges in the area.

The Lakenheath-based Pave Hawks are operated by the 56th Rescue Squadron, which moved to the U.K. from its former base at Keflavik, Iceland, back in 2006. The squadron has regularly deployed crews to Afghanistan but has also supported British authorities performing several long-range search-and-rescue missions.

The crash is the first fatal incident involving U.K.-based U.S. military aircraft in the British Isles since March 2001, when two F-15C Eagles crashed near the summit of the Ben Macdui mountain in Scotland during a low-level training mission.