LONDON - Three of the London suicide bombers staged a dry run to the UK capital just over a week before they blew themselves up on the transport network and killed 52 commuters, police said today.

Peter Clarke, head of London's anti-terrorist branch, said surveillance television footage showed three of the four bombers visited London on June 28 and stayed in the city centre for nearly four hours.

"The obvious suggestion is they possibly were conducting a reconnaissance on that day. We know that's part of terrorist methodology," said Clarke. "It was a dry run."

He said they travelled by train from Luton and arrived at Kings Cross station in London just before 9 am. They were then captured by closed circuit television cameras at Baker Street underground station and again at Kings Cross before 1 pm.

Two of the men were carrying rucksacks but there was nothing to indicate there were bombs inside.

Four British Muslims -- Shehzad Tanweer, Hasib Hussain, Mohammed Sidique Khan and Germaine Lindsay -- detonated home-made bombs concealed in rucksacks on three underground trains and a bus at around 9 am on July 7.

The three men on the dry run were Tanweer, Khan and Lindsay, whose bombs exploded on underground trains, while Hussain's blew up later on a London bus.

Security sources believe he too had planned to target a train but changed his plans when the line he wanted to use was shut and may have boarded another bus before blowing himself up.

The new evidence emerged after officers watched 80,000 surveillance recordings, spoke to 3,000 witnesses, examined 30,000 items and searched 15 locations including a landfill site as big as 18 Olympic-sized swimming pools.

Clarke said the bombs were peroxide-based explosives which had been made in a sophisticated "bomb factory" in a house in Leeds and transported to Luton in coolers.

"It was a clearly and carefully thought out and effective manufacturing facility that was there," he said of the house where the bombs were made, adding that detectives were not yet certain how the devices were detonated.

He also said a car used by three of the men to travel to Luton on the day of the attacks had two nail bombs in a rucksack hidden under the passenger seat.

However, he said there was nothing to indicate that there had been a fifth bomber who had failed to show up.

In a video tape aired on Monday, al Qaeda's second-in-command Ayman al-Zawahri claimed the group had carried out the bombings to strike at "British arrogance".

Clarke said the attacks had taken time to plan but it was unclear what support or international links the bombers had.

"I would be very surprised if there hadn't been some sort of terrorist planning going on for several months," he said. "There is still an awful lot to learn about what led to the attacks."