-- Three of the four bombers who killed themselves and 52 other people in a July 7 attack on the London transit system apparently scouted their route on a trip to the city 10 days earlier, police said Tuesday.

Officials at Scotland Yard said the three men -- Mohammed Sidique Khan, Shehzad Tanweer and Germaine Lindsay -- were spotted by officers as they reviewed thousands of hours of video surveillance tapes from train stations in London and the central English city of Luton.

Police released photos of the men, casually dressed, arriving at the Luton station the morning of June 28, with two of them carrying backpacks. Later in the day, they are seen at the King's Cross and Baker Street stations in London, apparently following the same route they took to stage the deadliest attack on British soil since World War II.

Hasib Hussain, who police believe detonated a bomb on a double-decker bus on that day, was not seen in the surveillance photos released Tuesday.

Khan, Tanweer and Hussain were British citizens of Pakistani descent; Lindsay was a Jamaican-born British resident. Police have said they believe all four were young Muslims who had become radicalized.

Earlier this month, the Arabic television network al-Jazeera aired a videotape of a man identifying himself as Khan who called the bombings a response to British policies in Iraq and the Middle East. "We are at war, and I am a soldier," he said.

In another videotaped message aired on al-Jazeera Monday, Ayman Zawahiri, al Qaeda's second-ranking leader, said the group was responsible for the attacks and had the "honor" of carrying them out. Earlier messages from Zawahiri had praised the attack but stopped short of asserting direct responsibility.

British Prime Minister Tony Blair has said he believes British foreign policy did not inspire the attacks.

In a statement issued Tuesday, police said Khan, Tanweer and Lindsay spent about three hours in London on June 28. Their visit, the statement said, "might suggest the suspects were carrying out reconnaissance of potential targets on the London transport system and checking the time of the journey they intended to take on the day of the attack."

Police said "other cases here and abroad" suggest that terrorists sometimes visit their targets before an attack.

Police said they reviewed tapes from June 28 after finding train tickets and receipts during searches of addresses linked to some of the suspects.

The Scotland Yard statement offered the most extensive assessment in weeks of the status of the police investigation into the bombings. It said police have reviewed more than 80,000 surveillance tapes, taken statements from more than 3,000 witnesses and searched 15 locations.

It said more than 1,000 pieces of evidence had been taken from a single apartment in Leeds, a city about 200 miles north of London where several of the presumed bombers lived. Police said the search of that apartment, where they believe the explosives used in the attacks were made, took longer than six weeks.

Police have charged four men in a failed second series of attacks on July 21 in which no one was injured. A fifth suspect is in custody in Rome, and the British government is seeking his extradition. Police are still investigating possible links between the attacks.

A June 28 videotape image released by police shows Shehzad Tanweer, left, Germaine Lindsay and Mohammed Sidique Khan at the Luton train station on what police believe was a trip to scout bombing targets.