Bullets are scattered around the scene of the US military helicopter crash in Norfolk, posing a hazard to members of the public, police have said.
Specialist teams are examining the site of the Pave Hawk crash in Cley next the Sea, an area the size of a football pitch.
Speaking near the scene, Chief Superintendent Bob Scully of Norfolk Police said: "We have currently cordoned off about 400 square metres of the marshland area.
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"It's not on the beach, although there are some bits of debris which are vulnerable to high tide."
Mr Scully told the press conference: "The crashed aircraft did contain ammunition.
"It is bullets, if you will, but those are scattered about that area ... and so the site is hazardous to members of the public and those people that would normally visit this area for birdwatching and other nature-interest activities, so for the present time we will be assisting and working with the military to ensure public safety by restricting access to that area."
Mr Scully said it was too early to say what caused the crash.
"At the present time the coroner, who is responsible for the investigation into the deaths, is carrying out a daylight assessment of the situation and is then arranging for the deceased to be removed from the site," he said.
Emergency services worked throughout the night with the help of military personnel and volunteers after the aircraft from RAF Lakenheath came down at a nature reserve in Cley next the Sea at around 6pm during a low-flying training exercise.
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.
He went on: "We are moving from a potential recovery operation to one of preserving the scene and carrying out an investigation.
"As you would expect, we in the UK police have expressed our condolences to our US colleagues for the loss of some of their crew. It's a desperately sad time. The US authorities have been responsible for notifying the next of kin of those who have sadly died."
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.
Inquiries into the cause of the crash, as well as the recovery of the wreckage and the second aircraft, are expected to take a number of days to complete, due to the geography and the munitions from the crashed helicopter.
A 400m cordon protects the crash site, and the A149 through Cley remains closed.
Details of the four crew members will not be released until next of kin have been informed and it is not believed that anyone in the surrounding area has been injured, police said.
The HH-60G Pave Hawk helicopter was on a training mission from RAF Lakenheath - a base for the US Air Force - which lies on the Suffolk Norfolk border, around 50 miles from the crash site.
A statement from the 48th Fighter Wing of RAF Lakenheath said: "US military officials are coordinating the recovery efforts with the UK police and the Ministry of Defence.
"The aircraft, assigned to the 48th Fighter Wing, was performing a low-level training mission along the coast when the crash occurred."
A derivative of the more famous Black Hawk helicopter, the Pave Hawk gets its name from the PAVE acronym standing for Precision Avionics Vectoring Equipment.
They are used for combat search and rescue, mainly to recover downed aircrew or other isolated personnel in theatres of war.
They have a four-man crew and can carry up to 12 troops. Typically, training flights would replicate as closely as possible real missions which would mean weapons and ammunition would be carried.