The helicopter, a Pave Hawk assigned to the 48th Fighter Wing based at RAF Lakenheath air base, was on a low-level training mission with another helicopter along the Norfolk coast when it went down in a nature reserve on Tuesday evening.
The cause of the crash, near the village of Cley next the Sea, was not known. The area is about 130 miles (210 km) northeast of London.
"A strict cordon remains in place around the nature reserve in Cley where a significant number of bullets from the crashed aircraft are scattered across the area," Norfolk Constabulary, the local police force, said in a statement.
"This is mainly on marshland although some debris which was close to the beach has been moved as it would be vulnerable to high tide," said Chief Superintendent Bob Scully.
He said walking and bird-watching, both popular activities in the reserve, had been suspended for the safety of the public.
"(Bullets) are scattered about ... so the site is hazardous to members of the public," Scully told reporters.
Footage filmed from just outside the police cordon showed the second helicopter - which landed safely to assist following the crash - in a flat landscape of long grass. What remained of the crashed aircraft was not visible.
Scully said police were examining the site on behalf of a coroner, who under English law is in charge of investigating the deaths, but would later hand over control of the area to air investigators from the British and US military.
As well as police, officials from the Ministry of Defence, Air Accident Investigation Branch and US Air Force were assessing the site.
"UNUSUALLY LOW"
Residents said they were used to the sound of low-flying military aircraft from RAF Lakenheath but had heard an unusually loud noise overhead shortly before the crash was reported. No one on the ground was known to have been hurt.
"It came over the house and helicopters are not usually that low," shopkeeper Sue McKnespiey told Reuters Television in a nearby village, Salthouse.
Earlier, the 48th Fighter Wing, which also flies F-15 fighter jets, confirmed the deaths of all four airmen on board and said their names would be released 24 hours after their next-of-kin had been informed.
"Please keep the airmen's families in your thoughts," RAF Lakenheath said in a tweet.
Their bodies would be removed from the site once the coroner was satisfied that all the evidence necessary to her investigation had been secured, Scully said.
A spokesman for 10 Downing Street, the office of British Prime Minister David Cameron, offered condolences to the families.
The Pave Hawk is made by Sikorsky Aircraft Co, a unit of United Technologies Corp.
According to the US Air Force website, it is a modified version of the Army Black Hawk and its primary mission is "to conduct day or night operations into hostile environments to recover downed aircrew or other isolated personnel during war".
