British police have released a chilling photograph of the four young men who bombed London last week which shows them trudging into a train station on the morning they detonated their explosives.

One newspaper said British officials had checked out one of the four last year but concluded he did not pose a threat. As the investigation into the bombings continued in Pakistan and Egypt, Scotland Yard detectives published the picture in a bid to jog memories and garner more information from the public about the men?s movements on the morning of the attacks.

The photograph, taken from CCTV footage, shows the men walking into a railway station in Luton, just north of London, to take a train to the capital. The four are dressed casually and look relaxed, with their hands in their pockets. Each carries a backpack, thought to contain the bombs which tore through London?s transport system during last Thursday?s rush hour, killing 55 people.

The time code on the picture shows it was taken at 7.21 a.m. (0621 GMT), 89 minutes before three of the bombs went off in quick succcession at three subway stations. The fourth blast tore apart a double-decker bus nearly an hour later. Three of the bombers were young British Muslims of Pakistani origin, while the fourth was a Jamaican-born Briton. Two of them were teenagers, one was just 22 and the other was 30.

Pakistani security forces have arrested six men in connection with the bombings ? the most recent in the eastern city of Lahore, where they detained two men on suspicion of having links with one of the bombers, Shahzad Tanweer.

Tanweer had visited Faisalabad and Lahore during two trips to Pakistan over the last two years. Pakistani intelligence sources say that in 2003 he met a man later arrested for bombing a church in Islamabad.

In Egypt, police have arrested a British-trained biochemist, Magdy Elnashar, and are questioning him about the attacks. But Egyptian Interior minister Habib el-Adli has said Elnashar was not a member of al Qaeda and that Western and Arab media had drawn hasty conclusions about the arrested man.

Looking for support network

The 33-year-old Egyptian was a researcher at Leeds University in England, and police are carrying out extensive searches at his rented house in the city, which was home to three of the bombers. Police say they have yet to establish beyond doubt that the bombers had intended to die in western Europe?s first suicide bombing, even if alternative theories seemed unlikely.

?We?ve never used the phrase ?suicide bombers?. We?ve always been aware that amongst the things we need to clarify is the notion these people intended to die as well as letting off a bomb,? a spokesman said on Saturday.

Some newspapers have suggested the bombers might have been duped into believing they had time to escape after planting the devices. The Sunday Telegraph said the men had bought return tickets from Luton to London and had even paid for a carpark ticket for the car they left at Luton station.

Police are looking for a support network of planners, bomb-makers and financiers behind the men. They expect to find clear links to al Qaeda, the militant Islamist network behind the Sept. 11, 2001, attacks on the United States and other bombings from Indonesia to Iraq and from Africa to Spain. The Sunday Independent newspaper said police had established a link between the oldest of the bombers, Mohammad Sidique Khan, and al Qaeda.

It said a Pakistani-American who is believed to have attended an al Qaeda ?summit? in Pakistan last year and who pleaded guilty to terrorism charges in the United States following his arrest shortly afterwards, had identified Khan from photographs. The Sunday Times, citing a senior government source, said British intelligence agency MI5 vetted Khan last year but concluded that he did not pose a threat.

Police are continuing to search houses in and near Leeds. They have so far raided 10 homes in the area and finished searching three of them. Officers in London have been given until tomorrow to question a 29-year-old man arrested in the raids on suspicion of ?the commission, instigation or preparation of acts of terrorism?.

Gideon LONG

10 days of horror and shock

● July 7, 2005 - Four blasts rip through London during the morning rush hour, killing at least 55 people. Seven hundred people are wounded. Bombs explode on three packed underground trains and another blast later strikes a double-decker bus.

A previously unknown ?Secret Group of al Qaeda?s Jihad in Europe? claims responsibility in the name of al Qaeda shortly after the blasts.

July 12 - Police suspect four men of carrying out the bombings. They are later revealed to be British nationals.

Police search six houses in and around the northern city of Leeds, including the homes of three of the four suspects.

Some 500 people are evacuated in the search and police arrest a relative of one of the suspects and seize large quantities of explosives.

Police also seize a vehicle in a car park in Luton, near London, finding some explosives.

● July 13 - Security experts say the four bombers would have received training and direction from a more senior militant for their attacks.

● July 14 - Thousands of Londoners hold a vigil in Trafalgar Square at the heart of the capital.

● July 15 - In Egypt, police arrest a British-trained biochemist, Magdy Elnashar, and are questioning him about the attacks.

Police have found highly volatile homemade explosive at sites linked to four suspected attackers, security sources say.

London police chief Ian Blair says detectives are confident of finding an al Qaeda link and warns that another attack is a strong possibility.

● July 16 - Pakistan says it has detained four men in Lahore and Gujranwala on suspicion of links with bombers.

Blair says the ?evil ideology? of al Qaeda must be pulled up by the roots as the death toll rises to 55.

● July 17 - Police release CCTV pictures of all four men walking into a railway station at Luton.