(CNN) -- Recovery of the bodies of the four U.S. Air Force personnel who died Tuesday evening when their military helicopter crashed near Cley, on the North Sea coast of England, could take more than a day, an official said.
"You would be very much mistaken if you thought that this would be a quick process," Norfolk Constabulary Chief Superintendent Bob Scully told reporters.
"A lot hinges on our ability to understand what happened to the aircraft that crashed, and that includes the detailed investigation that needs to be done."
The U.S. Air Force Pave Hawk HH-60G helicopter, from a British base in Lakenheath, crashed in a nature preserve.
The Royal Air Force base said the crash occurred during an evening training mission while the helicopter was flying low.
Cley is 180 kilometers (about 110 miles) northeast of London.
Removal of the dead from the aircraft "can disrupt the evidence and so it has to be done methodically, step by step ... possibly until tomorrow," Scully said.
Specialists from the Royal Air Force, the U.S. Air Force and investigators from the Norfolk Constabulary were working together to find out what happened, he said.
Scully said he did not know whether families of the dead had been notified.
Though ammunition scattered across the site during the crash, "the community do not need to be concerned" as long as they respect the cordon erected around the site, he said.
Roads near the site have been closed and will probably remain that way until Monday, he said.
According to the Air Force website, a Pave Hawk is a "highly modified" version of a Black Hawk helicopter that often carries a crew of two pilots, one flight engineer and one gunner.
CNN's Barbara Starr contributed to this report.