LONDON – British and FBI officials investigating the London terror attacks focused yesterday on an Egyptian-born chemist who studied in America and an 18-year-old Briton of Pakistani descent believed to have set off the bomb aboard a red double-decker bus.

Security forces in camouflage searched the Beeston area of the northern city of Leeds as police tried to crack the network thought to have given the dead suspects planning, logistical, and bomb-making support.

“We don’t know if there is a fifth man, or a sixth man, a seventh man, or an eighth man,” London’s Metropolitan Police commissioner, Sir Ian Blair, told foreign journalists.

British authorities were seeking a Pakistani Briton with possible ties to Al Qaeda followers in America, news reports said. They said he may have organized the attacks and chosen the targets, leaving Britain the day before the July 7 bombings.

“Al Qaeda is not an organization. Al Qaeda is a way of working … but this has the hallmarks of that approach,” Mr. Blair said of the attacks, which killed 53 people, including four bombers. “Al Qaeda clearly has the ability to provide training … to provide expertise … and I think that is what has occurred here.”

FBI agents in Raleigh, N.C., joined the search for the chemist, Magdy Asi el-Nashar, a 33-year-old former North Carolina State University graduate student. The doors were locked yesterday at the building at Leeds University where he recently taught chemistry.

And in a further international development in the inquiry, Jamaica’s government said it was investigating a Jamaican-born Briton as one of the bombers.

Britain paid tribute yesterday to those killed in the attacks with two minutes of silence. Office workers spilled out into the streets, construction crews put down their tools and held hard hats in their hands, and London’s famous black cabs pulled to the side of the road.

Queen Elizabeth II stood motionless outside Buckingham Palace, and a crowd, many wiping away tears and bowing their heads, filled Trafalgar Square.

Trafalgar is about a mile and a half from Tavistock Square, where Hasib Hussain, 18, allegedly set off the bomb that killed 13 people aboard the bus. That blast occurred nearly an hour after three London Underground trains blew up, and investigators don’t yet know what Hussain did during that hour or when he boarded the bus.

Trying to map out Hussain’s movements, police appealed for information from anyone who may have seen him in or around King’s Cross station, where the four suspects parted ways. They released a closed-circuit television image showing him wearing a large camping style backpack as he strode through a train station in Luton, outside London, about two and a half hours before he allegedly blew up the no. 30 bus. He had a mustache and wore jeans, a white shirt, and a dark zip-up top or jacket.

A separate photo of his face showed him with a beard, looking straight ahead.

“Did you see this man at King’s Cross?” the head of the Metropolitan Police’s anti-terrorist branch, Peter Clarke, asked in a televised appeal. “Was he alone or with others? Do you know the route he took from [King’s Cross] station? Did you see him get onto a no. 30 bus?”

The young men traveled together from Luton to King’s Cross just before the blasts, police said.

Police officially identified two of the suicide bombers Thursday, Hussain and Shahzad Tanweer, 22, whom they say attacked a subway train between Liverpool Street and Aldgate stations.

Both were Britons of Pakistani descent, as was Mohammed Sidique Khan, 30, identified by the press as one of the bombers. Reports say the fourth attacker was Jamaican-born Briton Lindsey Germaine.

A Jamaican Foreign Affairs Ministry spokesman, Wilton Dyer, said officials were waiting for Britain to confirm the identity of the suspect before they could help in identifying his possible origins in Jamaica. “They [British officials] say they are not in a position to identify this man, they need more forensics,” Mr. Dyer told the Associated Press.

Mr. Blair said finding those who planned the attack “is the absolute focus of the current investigation.” An outside mastermind may have recruited the four bombers, provided explosives, helped build the bombs, or given other logistical support.

The Times of London said investigators believe a Pakistani Briton in his 30s with possible links to Al Qaeda may have orchestrated the attacks. They believe he arrived in Britain last month and left just ahead of the bombings, the newspaper said.

It reported that the man, whom it did not identify, was thought to have chosen the targets. There has been speculation that the bombers intended to hit four subway trains, but that Hussain got on the bus instead because one Underground line had been halted by mechanical problems.

The Times said detectives also want to locate Mr. el-Nashar, who was thought to have rented one of the homes police searched in Leeds in a series of raids Tuesday. Neighbors reported Mr. el-Nashar recently left Britain, saying he had a visa problem, the newspaper said.

A North Carolina State University spokesman, Keith Nichols, said a person named el-Nashar studied there as a graduate student in chemical engineering for a semester beginning in January 2000.

The head of NCSU’s chemical engineering department, Peter Kilpatrick, said he handed over all his files on Mr. el-Nashar to FBI agents yesterday.

Members of the FBI’s Joint Terrorism Task Force in Raleigh were working on the case, said Michael Saylor, who heads the Raleigh FBI office. He referred other questions to FBI headquarters in Washington, which declined comment.

While authorities searched for the chemist and the Pakistani Briton, British police were questioning a 29-year-old man they arrested in the Leeds raids. Britain’s Press Association news agency has identified him as a relative of a suspected bomber.

Two of the attackers had brushes with the police before the bombings, and one had been linked loosely to another terror plot, news reports said.

Tanweer was reportedly arrested once for shoplifting, and Hussain had been questioned for disorderly behavior.

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 half a ton of a chemical fertilizer used in many bombings, ammonium nitrate.

Counterterrorism officials have looked at whether the bombers in last week’s attacks used plastic explosives, said one official who spoke on condition of anonymity because the investigation is ongoing. Among those considered, the official said, is a compound called TATP, or triacetone triperoxide.

In 2001, Richard Reid used an improvised shoe bomb rigged with TATP, which is difficult for bomb-sniffing dogs to detect, when he boarded an airplane and tried to blow it up over the Atlantic.

TATP is a highly unstable explosive made from commercially available chemicals such as acid, acetone, and peroxide. Plastics are a broad category of explosives, some with military or commercial uses. TATP, however, is considered too unstable for such purposes.