Weapons experts are to analyse in "minute detail" an improvised bomb attack on a patrol boat that killed four UK servicemen in southern Iraq.
Another three were seriously hurt on the Shatt al-Arab in Basra on Sunday.
The dead were from the Royal Signals, the Intelligence Corps, and two from 539 Assault Squadron Royal Marines, one of them attached from 45 Commando.
The Ministry of Defence had earlier said both marines were from 45 Commando Royal Marines.
The condition of one of the injured has improved significantly but the other two remain in a very serious condition.
Some of the families of those killed have asked for a period of 24 hours before their names are released, and the MoD anticipates the dead will be named on Tuesday morning.
539 Assault Squadron is based at Turnchapel, near Plymouth in Devon.
Captain Tane Dunlop said an inquiry involving a wide information-gathering network was already under way.
It would try to find out how insurgents were able to target the boat, he said.
The three injured troops were taken to a field hospital where they underwent surgery.
A total of 125 British troops have now died in Iraq, 95 from hostile action.
Foreign Secretary Margaret Beckett said: "An investigation is being undertaken now as to exactly what happened, and whether it poses a significant new threat, but obviously it is very tragic and everybody feels very sorry indeed for the families involved."
Defence Secretary Des Browne has again extended his condolences to the families of the British servicemen who were killed.
He told BBC Radio 4's Today programme: "My thoughts are with them and also with their colleagues who will continue I'm sure to show the professionalism that they've shown throughout the time that they've been there.
"This terrible incident yesterday just shows the nature of the challenges that we put to these people day in, day out, and have done for some time now and I accept that's a very difficult position for them."
The attack took place at 0950 GMT (1250 local time) and is thought to be the first such attack on a patrol boat carrying British personnel.
They have been patrolling the waterway, which borders Iran, since 2003.
It was thought to be a safer way to travel between bases than going by road.
Capt Dunlop said: "The use of improvised explosive devices is very common in Iraq, more often further north but there have been a number of such attacks in the Basra province and Basra city.
"It is slightly unusual in that this time it was targeting a boat, but they're usually used against any form of multi-national forces or personnel."
The BBC's David Loyn, in the Iraqi capital Baghdad, said the attack was an extension of the war to water.
Basra, Iraq's second city, is located on the Shatt al-Arab waterway
"If insurgents are able to operate on the waterway against the military then that does open a new front," he said.
He added that there was an attack a couple of years ago on American forces further out in the Gulf but this was the first such incident on the waterway itself.
On the same day, two suicide bombers killed 35 people and wounded 60 at a police commando recruiting centre in western Baghdad, police said.
And the US military said three American soldiers were killed in combat in western Al-Anbar province.
The deaths came on the same weekend that troops in Basra and elsewhere in Iraq paid their respects to lost colleagues, to mark Remembrance Day.
The Iraqi ambassador to the UK, Salah Al-Shaikhly, offered condolences to the families of the dead and injured.
Liam Fox, the shadow defence secretary and Sir Menzies Campbell, leader of the Liberal Democrats, expressed their sympathy to the families of those killed.