THE human cost of Britain's worst mainland terrorist attack became clear last night as desperate friends and relatives searched London for missing loved ones.

Police admitted the official death toll of 49 would inevitably rise as they continued the grim task of recovering bodies.

Last night, teams were still removing corpses from King's Cross station and trying to establish how many people were on the double-decker bus destroyed in Tavistock Square.

Scotland Yard vowed to track down the terrorists behind the four explosions that rocked the capital on Thursday.

They believe the attacks were the work of a terrorist cell rather than one individual, and officers fear the cell could still be active.

Commissioner Sir Ian Blair said the bombings bore all the hallmarks of al Qaida, in that they were near-simultaneous attacks with no warning, causing mass casualties.

He said more than 50 people had died in the four blasts and 700 were injured, including people from Australia, China, Poland, Portugal and Sierra Leone.

A huge hunt for the perpetrators, involving intelligence agencies from around the world, is already under way.

Meanwhile, relatives and friends of people missing since yesterday's bomb blasts began an increasingly desperate attempt to find their loved ones.

Mothers, fathers, husbands, boyfriends and girlfriends walked the streets of London. In a chilling echo of the 9/11 terror attacks on the US, many carried photographs in the hope someone would recognise the pictures and provide crucial information.

Although they all vowed not to give up, many must now fear they will never see their loved ones alive again.

At King's Cross scores of floral tributes were laid.

One card, borrowing from the Ralph McTell song Streets Of London, read: "So you think you can defeat us? Let me take you through the streets of London and I'll show you something that will make you change your mind."

Meanwhile, anti-terror police began sifting through hundreds of hours of closed-circuit television (CCTV) footage, interviewing thousands of witnesses, reviewing recent intelligence and gathering forensic evidence.

A senior security source said: "There is a huge amount of work going on. Everything you would expect to be done is being done.

"If there is any opportunity of intervening, of trying to lessen the risk to the public, then we will take that."

Analysis of the CCTV footage from Tube stations and from along the route taken by the number 30 bus, which exploded in Tavistock Square, is likely to be one of the key lines of inquiry.

"CCTV is absolutely crucial here," the source said.

"Tubes run through many stations and the bus will have followed its route. The people who did this could have got on at any number of places. This is probably the largest CCTV recovery we have ever had to do."

Another key element is to follow up the thousands of calls the anti-terrorist hotline has received since the bombings. At one point, police said they were taking 42,000 calls an hour.

Forensic experts will also have a key role to play. Detailed examination of the train carriages has begun to determine the type of explosive used in the blasts.

They have already established that each device used less than 10lbs of high explosive and was probably of a size that could have been carried in a bag or a rucksack.

Those on the Tube trains appeared to have been on the floor of the carriages when they exploded.

Officers said they would not describe the devices as sophisticated, but it was clear the bombings involved "a degree of planning".

The bus that was ripped apart in Tavistock Square will eventually be taken away from the scene for detailed forensic analysis.

Police have so far refused to give any indication about whether they believe that explosion - or any of the blasts - was the work of a suicide bomber.

A source said the bombs could have been detonated by fuse, by timer, or remotely and nothing had as yet been excluded.