A US military helicopter crash site poses a risk to the public because of ammunition scattered across the area, police have said.
Norfolk Police Chief Superintendent Bob Scully said debris from the crash - which killed four crew members - is spread across an area of difficult terrain "the size of a football pitch" in Cley Next the Sea.
He said a cordon will remain in place for most of the day while investigators carry out a full examination and work to preserve evidence.
He said: "The crashed aircraft did contain ammunition. This is not of any great significance - it is bullets, if you will - but it is scattered across the area.
"The site is a hazard to members of the public and people who would normally visit for birdwatching and other nature activities."
The Pave Hawk helicopter - based at RAF Lakenheath in Suffolk - was taking part in a low-flying exercise when it came down at around 7pm on Tuesday.
Mr Scully said the coroner is carrying out a daylight assessment of the area and will then arrange for the victims' bodies to be removed from the site.
The investigation will then be passed over to the Air Accidents Investigation Branch and its counterparts in the US, he said.
Emergency services worked at the scene throughout the night with the help of military personnel and volunteers.
A second helicopter from RAF Lakenheath was also in the area at the time of the crash and set down on the marshes to try to assist.
Mr Scully refused to speculate on whether the second helicopter had any involvement in the incident but said that, as it was nearby at the time, it made sense that it went to help.
Residents told of hearing a "heavy and very unusual" sound overhead as the helicopter - which specialises in recovering troops from war zones - plummeted into marshland at the Norfolk Wildlife Trust Cley Marshes Nature Reserve.
The Pave Hawk - a derivative of the more famous Black Hawk - gets its name from the PAVE acronym standing for Precision Avionics Vectoring Equipment.
The helicopter is used for combat search and rescue, mainly to recover downed aircrew or other isolated personnel.
It usually has a four-man crew and can carry up to 12 troops.
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