British authorities investigating the attacks on London's transit system were searching Thursday for an Egyptian-born academic who recently taught chemistry in Leeds, the city in northern England where three of the suspected bombers lived, officials said.

British newspapers reported that Magdy Nashar, 33, had helped a friend rent an apartment in Leeds where police found explosive material earlier this week. Authorities said they had no evidence that Nashar helped in the attacks, which killed at least 54 people, but they wanted to question him. His whereabouts are unknown.

Nashar attended North Carolina State University for a semester in 2000, and the FBI confirmed Thursday that, at British officials' request, field agents were seeking to piece together his stay in the United States.

With the four bombers who perpetrated the attacks apparently dead, police were seeking to talk to anyone who might have knowledge of the plot or of people involved in it.

British investigators have said they now believe that one of the men who carried out the morning rush-hour attacks on three subway trains and a double-decker bus was a Jamaican-born Briton named Lindsey Germaine. Earlier reports had suggested that all four were British-born men of Pakistani descent.

Police searched Germaine's last-known residence in Aylesbury, a town about 40 miles northwest of London, according to British security officials.

On a day when the country paused at noon for two minutes of silence to honor the dead and injured, Britons got their first look at images of the youngest of the alleged bombers: Hasib Hussain, 18, who investigators say they believe blew up a No. 30 bus nearly an hour after the three synchronized explosions on subway trains.

Police released two photos, one of them a chilling image from a security video camera at the Luton train station north of London that showed a solemn Hussain carrying a large backpack at 7:20 on the morning of the bombings. Police say he and his three companions were on their way to King's Cross station in London hauling backpacks full of high-grade explosives.

Authorities pleaded for witnesses to come forward if they could describe Hussain's movements on the morning of the bombings and help explain why he bombed the bus rather than another subway train.

"Did you see this man at King's Cross?" Peter Clarke, head of the Metropolitan Police anti-terrorist branch, said at a news briefing in remarks directed at the public. "Was he alone or with others? Do you know the route he took from the station? Did you see him get onto a No. 30 bus?"

Officials for the first time publicly confirmed the names of two suspected bombers -- Hussain, who they have concluded died on the bus, and Shehzad Tanweer, 22, who died in the subway train explosion at Aldgate station. They said that the bodies of both men had been identified but that police had not conclusively identified the body of a third, named by friends as Mohammed Sidique Khan, 30.

All three were British citizens of Pakistani origin who lived in Leeds, about 200 miles north of London. Police say they believe that the three and Germaine intended to kill themselves as well as their victims in what was Western Europe's first suicide bombing.

Officials said DNA and other evidence made it appear very likely that Germaine was responsible for the explosion that rocked the Piccadilly Line subway train near Russell Square station and that he died in the blast. But they gave no further details about who he was or how he was connected to the three other suspects.

Nashar's name surfaced in British newspapers Thursday morning. According to the Daily Mirror, he approached the owner of a Leeds flat about a month ago to arrange for a "friend from London" to live there. The owner, Shakir Al Ani, a local physician, said he realized there might be a connection to the bombings only over the weekend, after his phone number was found in the cell phone of one of the suspected bombers.

Nashar told the physician two weeks ago that he was leaving for Egypt, saying he was having visa problems, the newspaper reported. The Mirror said it was unable to contact Nashar but noted there had been no suggestion that he had knowingly helped the bombers. Police have not characterized him as a suspect.

Keith Nichols, a spokesman for North Carolina State in Raleigh, confirmed that Nashar attended the school in 2000 for the semester that started in mid-January and ended in early May. Nashar took a graduate class in chemical engineering, and his last known address was in university housing, Nichols said.

Dean Boyd, a spokesman for the Immigration and Customs Enforcement division of Homeland Security, said the service was working closely with the FBI "to investigate any and all leads that may be related to the investigation by British authorities."

An FBI official who spoke on condition of anonymity said it was too soon to tell what Nashar's role, if any, might have been, or whether he had contact with suspected extremists while he was in the United States.

Leeds University said in a statement that Nashar enrolled there in October 2000 to do biochemical research. He was sponsored by the National Research Center in Cairo and earned a doctorate at the university, the statement said. A university spokeswoman was quoted by the Guardian as saying he had not been seen on the campus since the beginning of July.

On Thursday, police sealed off the Beeston area of Leeds where Tanweer lived and evacuated hundreds of residents, two days after they first raided six houses in search of evidence and explosives. "They said there might be a bomb," said Sarah Crosby, 22, who said a policeman knocked on her door at 10:30 a.m. and told her to be prepared to be away from home for at least two days.

Clarke said the fast-moving investigation -- in which detectives have taken more than 500 witness statements and analyzed more than 5,000 security camera videotapes -- had uncovered a vast amount of information but was still in its early stages.

Some of the leads that investigators are following include a possible connection between the suspected bombers and Islamic extremists in Pakistan, to which at least two of the suspects had traveled in recent years, security officials said. They are also exploring a possible link between the bombings and last year's roundup of nine men on suspicion of planning a fertilizer bomb attack in London.

Investigators are also looking at the possibility that the bombs were smuggled into Britain from a Balkan state where military-grade explosives are available on the black market. A police official who declined to be named because of the sensitivity of the probe cautioned that these leads remained unproved theories and were among many being pursued.

London became a solemn still-life portrait for two minutes starting at noon. Workers poured quietly out of office buildings and stores across the city just before the hour and stood silently as Big Ben tolled. Queen Elizabeth, dressed in pink, stood in the courtyard outside Buckingham Palace.

At King's Cross, conductors halted their trains and joined passengers on the platform. People turned off their cell phones and bowed their heads. Others throughout Britain and in many Western European capitals joined in the commemoration.

At a bus depot in East London, George Psaradakis, the driver who survived the bombing of the No. 30, paid tribute to those who died. "I send my thoughts at this time to the families of the innocent victims, especially my fellow colleague whose daughter lost her life on my bus," he said. "In today's silence we remember them with quiet dignity and respect, and we show our deep contempt for those who planted the bombs and those who masterminded them."

Correspondent Craig Whitlock in Leed and staff writers Dan Eggen and Darryl Fears in Washington and Tamara Jones in London contributed to this report.

Police released image of Hasib Hussain, 18, a bombing suspect.