Four people were killed when a US air force helicopter crashed during a “low-level” training exercise.
The aircraft came down in Cley next the Sea in north Norfolk. Emergency services are at the scene with a 1,200ft (400m) area cordoned off.
The helicopter is an HH-60G Pave Hawk, based at RAF Lakenheath in Suffolk, a spokesman for the USAF confirmed.
The aircraft usually carries a four-man crew on board, he said.
Police said residents can stay in their homes but pedestrians and motorists are being diverted away as there is live ammunition on board.
An RAF Lakenheath spokesman said: “The aircraft, assigned to the 48th Fighter Wing, was on a low-level training mission when the crash occurred.”
Six appliances from Norfolk Fire Service attended.
The helicopter came down at 18:00 GMT and a spokesman said the first unit arrived at the scene at 19:53 GMT.
Richard Kelham, chairman of Cley Parish Council, said the helicopter had crashed in the middle of Cley Marshes, a bird reserve.
The helicopter came down on marshes that were badly flooded during last month’s tidal surge.
The cause of the crash remains a mystery. The weather conditions are good and seem unlikely to have been a factor in what happened.
Police are assessing the scene and the area is likely to remain cordoned off for some time. Further clues are likely to emerge at first light.
“The incident is very sad,” he added.
Suzanne McKnespiey, who runs Cookies Crab Shop, in nearby Salthouse, said she heard the sound of a low aircraft overhead between 18:30 and 19:00.
“I quickly looked out of the window and it was a helicopter,” she said.
“It was low and very loud. It flew overhead and out towards the sea, going very fast.
“We’re used to hearing helicopters here but not that low.”
She said she was shocked to hear the news of the four deaths.
“It’s terrible. It’s very sad and makes you feel sick to the pit of your stomach,” she said.
“It makes you realise all your troubles are little ones.”
Her husband Peter said he heard what he thought were two helicopters passing overhead.
“One sounded not quite right. The next thing I heard was a siren, maybe 10 minutes later but I didn’t think much more of it.
“Then there was a lot of activity. Blue flashing lights and sirens.”
He said he understood the crash had taken place near a raised bank known as East Bank, above the flooded marshland.
Michael Girling, who was also in the area, said he heard the impact of the crash.
“I thought the helicopter had landed on the beach,” he said. “It had obviously ditched.”
Brendan Joyce, chief executive officer of Norfolk Wildlife Trust, said the nature reserve would be closed on Wednesday.
It was initially thought the aircraft had ditched in the North Sea.
A spokesman for the Royal National Lifeboat Institution said three crews were launched before being stood down soon after when it was established the aircraft was onshore.
The 48th Fighter Wing, also known as the Liberty Wing, is assigned to the United States Air Forces in Europe (USAFE).
In addition to HH-60G Pave Hawks, it is home to squadrons of F-15 Eagle tactical fighter planes and F-15E Strike Eagle dual-role fighters.
Military aviation analyst Roger Smith said he was “very surprised” this type of helicopter was involved in a crash.
“It’s a very safe outfit… well equipped to fly in poor weather with a highly trained crew, used to flying at night,” he said.
He said the crew would normally be made up of a pilot, co-pilot, crew chief (flight engineer) and gunner.
“It’s main role is combat air rescue when an allied pilot is shot down behind enemy lines,” he said. “This has to be done in the cover of darkness so they have to be able to fly in the dark in bad weather.”