The investigation into the deaths of six British soldiers in southern Iraq will take days, the Ministry of Defence has said.
It was unclear who fired the first shot at the police station
The men, all members of the Royal Military Police, were shot in a village near Amara, on Tuesday.
It is unclear what happened, although local residents said the killing was by civilians unhappy with the way homes were being searched for weapons.
It is the heaviest single hostile combat loss for British forces since the 1991 Gulf War, and could prompt a rethink of military tactics.
There are 14,000 British forces patrolling parts of Iraq in the aftermath of April's conflict, but "thousands" more could be sent if necessary, Defence Secretary Geoff Hoon said.
BRITISH TROOPS UNDER FIRE
Tuesday 0730 BST: Two British vehicles attacked on patrol in Majar al-Kabir by Iraqi gunmen who injure one soldier. A Chinook arrives in support and a further seven paratroopers are injured
1200 BST: Six military policemen found dead at police station in the same town
A senior British officer described the attack as "unprovoked murder".
A spokesman for the Ministry of Defence said there were no surviving British personnel to give accounts of the attack.
"We are aware of the differing reports and we are trying to find all these [Iraqi]
witnesses," he added.
"We are having to build a picture of what happened and the whole sequence of
"People are working very hard on this and hopefully it will become
clearer but that could take a few days."
Local Iraqi residents told BBC correspondent Clive Myrie they were angry at the way soldiers were searching their houses for weapons.
He said "scores" of Iraqis had stormed the local police station, killing the British servicemen and then torching the room they made a stand in.
Lieutenant Colonel Ronnie McCourt said: "The enemies of peace have claimed the UK forces are conducting violent searches of Arab homes and have not respected property. This is simply not true."
RMP TROOPS KILLED IN IRAQ
Corporal Simon Miller, 21
Sergeant Simon Alexander Hamilton-Jewell, 41
from Chessington, Surrey
Corporal Russell Aston, 30
Corporal Paul Graham Long, 24
Lance-Corporal Benjamin John McGowan Hyde, 23
Lance-Corporal Thomas Richard Keys, 20
Bala, N Wales
He later told the Reuters news agency: "This attack was unprovoked. It was murder."
One local community leader told the BBC that in recent days troops had ignored a written agreement to give adequate notice before they searched homes for weapons.
The community leader said hundreds of people had protested in front of the local police station on Tuesday. The soldiers fired shots and the crowd fired back before attacking the building he claimed.
Prime Minister Tony Blair told the Commons tension between British troops and Iraqis reluctant to disarm could have led to the killings.
He paid tribute to the dead soldiers, saying they had been doing "an extraordinary and heroic job trying to provide a normal and decent life for people in Iraq".
The six soldiers were among a contingent of officers sent to the country to help rebuild its police force.
All of the dead belonged to 156 Provost Company, part of the 16th Air Assault Brigade, based in Colchester.
Their bodies were found at a police station in Majar al-Kabir, about 100 miles north of Basra.
Some reports said up to four Iraqis were also killed.
In a separate attack in the same area, seven British soldiers were injured when their helicopter came under fire. Two of the casualties were seriously injured.
Mr Hoon told BBC Radio 4's Today programme there were "many thousands" of troops which could be sent to Iraq if deemed necessary.