Police also confirmed the identities of two men they believe attacked London’s transport system.
Peter Clarke, head of the Metropolitan Police anti-terrorist branch, said Shahzad Tanweer, 22, was responsible for attacking a subway train between Liverpool Street and Aldgate Stations, and that Hasib Hussain, 18, was responsible for the blast that killed 14 people on a double-decker bus.
Earlier, Metropolitan Police Commissioner Ian Blair conceded the four suspects were suicide bombers.
“You don’t need to be a suicide bomber in a liberal democracy. They have chosen to be,” he told the Foreign Press Association in the first public admission that the bombers were suicide attackers.
Police also were searching for an elusive fifth suspect, who they believe arrived in Britain last month and left the day before the bombings, British media reported Thursday.
Detectives think the suspect is a Pakistani Briton in his 30s with possible links to al-Qaida followers in the United States, according to Britain’s Press Association and The Times newspaper.
A British security official told The Associated Press that British intelligence and security services were working closely with Pakistani counterparts because of the suspects’ links with the country.
Along with Tanweer and Hussain, British media identified a third British suspect of Pakistani descent – Mohammed Sidique Khan, 30 – as well as Jamaican-born Briton Lindsey Germaine, as the suicide bombers who set off the July 7 explosions that killed 54 people.
Blair declined to comment on those reports, nor would he say how many suspects were being sought by police.
“We don’t know if there is a fifth man, or a sixth man, a seventh man,” he said, but added that police were trying to determine which organization or people had planned and supported the attacks.
Commenting on the possible role of al-Qaida, Blair said “al-Qaida is not an organization. Al-Qaida is a way of working … but this has the hallmark of that approach.”
“Al-Qaida clearly has the ability to provide training … to provide expertise … and I think that is what has occurred here,” the police commissioner said.
The Times said the purported mastermind was thought to have chosen the four bombing locations on London’s Underground. A red double-decker bus was also blown up, but many believe that attacker planned to hit a train on the subway’s Northern Line and was thwarted because mechanical problems shut the line down.
Blair said police believe “that we know who the four people carrying the bombs were … and we believe they are all dead.”
British Foreign Secretary Jack Straw declined to comment Thursday on the investigation, saying only: “there is intensive and increasingly substantial co-operation between the government of Pakistan and the United Kingdom government on counterterrorism, both in general and in specific cases.”
Straw acknowledged concerns that some of Pakistan’s thousands of religious schools, or madrassas, could be breeding grounds for Islamic militants. Pakistan’s President Gen. Pervez Musharraf has called on provincial authorities to monitor clerics closely to ensure they don’t use mosques to stoke sectarian violence.
“Yes, we are concerned about what goes on in some, though not all, of the madrassas in Pakistan, but so, too, is President Musharraf” and the Pakistani government, Straw said.
On Tuesday, police raided six homes, including those of three suspected bombers, around the northern city of Leeds . Officers hunted for explosives and computer files that could lead to the mastermind behind the attacks. They arrested a 29-year-old man, identified by Press Association as a relative of one of the suspected bombers, and a judge approved his detention through Saturday.
Reports said Tanweer had been arrested once for shoplifting, and Hussain was once questioned for disorderly behaviour.
The Independent newspaper, citing police sources, said one suspect – it did not say which – had been linked loosely to a plot to build a large bomb near London. It said police described the link as a low-level “association.”
That appeared to be a reference to a ring cracked in March 2004, when eight men were arrested across southern England in an operation that led to the seizure of almost half a tonne of ammonium nitrate, a chemical fertilizer used in many bomb attacks. Several have been charged and face trial.
The Guardian newspaper suggested that the mobile phone number of one of the suspected bombers was found during an investigation.
Police have closed-circuit television footage of the four young men carrying backpacks and arriving at King’s Cross station in central London 20 minutes before the subway explosions. The Evening Standard said police had spotted a fifth man with the group on the tape.
The Times said detectives wanted to locate Magdy Asi el-Nashar, 33, an Egyptian-born academic who recently taught chemistry at Leeds University. The paper said he was thought to have rented one of the homes searched in Leeds.
In North Carolina, FBI agents joined the search for el-Nashar at North Carolina State University.
El-Nashar studied chemical engineering at the university for just one semester beginning in January 2000, university spokesman Keith Nichols said.
Peter Kilpatrick, the head of NCSU’s chemical engineering department, said he handed over all his files on el-Nashar to FBI agents Thursday.
Neighbours said el-Nashar recently left Britain, saying he had a visa problem, the Times said.