News reports said authorities in Britain identified the fourth suspect Thursday in last week's deadly London attacks and police acknowledged the blasts were homicide bombings on the same day the nation silently paid tribute to the victims.
Sky News said police identified the fourth bomber as Lindsey Germail, a Jamaican-born Briton in his 30s who may have been responsible for the blast at King's Cross station. Metropolitan Police Commissioner Ian Blair (search) declined to comment on the report.
However, Blair confirmed to the public for the first time that police believe all four blasts were homicide bombings.
"You don't need to be a suicide bomber in a liberal democracy. They have chosen to be," Blair told the Foreign Press Association.
Blair said police believe "that we know who the four people carrying the bombs were ... and we believe they are all dead."
"We are as certain as we can be that four people were killed and they were the four people carrying bombs," Blair added.
Commenting on the possible role of Al Qaeda, Blair said, "Al Qaeda is not an organization. Al Qaeda is a way of working ... but this has the hallmark of that approach."
"Al Qaeda clearly has the ability to provide training ... to provide expertise ... and I think that is what has occurred here," Blair said.
Reuters reported that Germail was linked to a house police searched in Aylesbury on Wednesday evening.
Police on Thursday raised the death toll from 52 to 54.
Reports also said police were focusing on a fifth suspect in the attacks who is said to be a British suspect and the possible mastermind of the attacks.
Meanwhile Thursday, office workers wordlessly filled the streets, construction workers removed their hard hats and London's famous black cabs pulled to the side of the road as Britain silently paid tribute to the 54 dead and 700 wounded from four terrorist bombs that struck a week ago Thursday.
In Leeds, another house was being searched in the Beeston area. Sources said detectives are hunting two other people known as the "mastermind" and the "chemist" in the attacks.
The Times newspaper, quoting unidentified police sources, said detectives were interested in locating Magdy el-Nashar, 33, an Egyptian-born academic who recently taught chemistry at Leeds University. The Times said he was thought to have rented one of the homes being searched in Leeds.
Neighbors reported that el-Nashar had recently left Britain, saying he had a visa problem, The Times reported.
In the U.S., North Carolina State University spokesman Keith Nichols said el-Nashar was enrolled at the school for a semester beginning in January 2000 as a graduate student in chemical engineering.
Saad Khan, the chemical engineering department's director of graduate programs, said he remembered that el-Nashar applied for admission while living in Egypt.
But by the end of the spring semester, el-Nashar had changed direction and decided to pursue a doctorate at Leeds instead, Khan said.
A military bomb squad cordoned off half a block of the Beeston area of Leeds as unit members in camouflage fatigues expanded their systematic search of the neighborhood.
One police officer, who refused to give his name, told The Associated Press the officers were going block-to-block to try to speak with anyone who might have known the suspects. Police were also making a quick examination of select homes, looking around for anything they might find suspicious.
On Thursday police held a news conference where they showed a composite photo of Hasib Hussain (search), suspected of detonating the blast that destroyed the No. 30 bus in Tavistock Square.
Police issued a plea for Londoners who may have seen him during the time he arrived at King's Cross station, where he was captured on closed-circuit TV, and the time he boarded the No. 30 bus, which exploded at Tavistock Square. One image showed Hussain walking in the Underground station and carrying a large backpack.
The Daily Telegraph said police were trying to identify a man seen standing near the four suspects on the platform at Luton railway station, where they apparently boarded a train for London on July 7.
The Evening Standard reported that police had spotted a fifth man on closed-circuit tape showing the group at King's Cross about 20 minutes before the explosions.
Late Wednesday, Scotland Yard said anti-terror police had raided a residence northwest of London.
Officers carried out a forensic examination, but police would not say why they targeted the house in a quiet residential street in Aylesbury, about 40 miles from London and 20 miles from Luton — where a vehicle thought to be linked to last week's attacks was towed away Wednesday.
News reports have identified three Britons of Pakistani descent as suspects in the attacks.
A U.S. government official confirmed that Shahzad Tanweer (search), Hasib Hussain and Mohammed Sidique Khan (search) are believed to have been three of the bombers. The official spoke on condition of anonymity because British investigators have not publicly released the identities of the suspected attackers.
Anti-terrorist branch head Peter Clarke said Shahzad Tanweer attacked a subway train between Liverpool Street and Aldgate stations, and Hussain blew up the double-decker bus, killing 13.
Surveillance cameras captured the four as they arrived in the capital 20 minutes before the rush-hour explosions began.
A British security official told the AP on Thursday that Britain's intelligence and security services were cooperating closely with their Pakistani counterparts because of the suspects' links with the country.
British media said Tanweer was a 22-year-old cricket-loving sports science graduate; Hussain, an 18-year-old; and Khan, the 30-year-old father of an 8-month-old baby.
The British Broadcasting Corp. has reported that a fifth suspect is being sought in Western Europe's first homicide bombing attack, citing unidentified sources. Police refused to comment.
Authorities suspect the attackers didn't work alone, and that their collaborators or leader are likely still at large.
Britain's top law enforcement official, Home Secretary Charles Clarke said authorities were looking closely "at the relationship between the people who may have committed the offenses and the wider network around them."
Several officials, including Foreign Minister Jack Straw, have said the attacks bore the "hallmark" of Al Qaeda. Two claims of responsibility purportedly from militant Islamic groups have surfaced.
Police continued on Thursday to detain a 29-year-old man, arrested during raids on Leeds on Tuesday. In London, officers resumed the task of searching through voluminous evidence from closed-circuit TV footage and the grisly scene where the blasts ripped apart three trains and a bus.
An uncle of Tanweer said his nephew had gone to Pakistan for two months earlier this year to study religion, and that the family believed he was attending "some religious function" on the day of the bombings.
"It was total shock. I mean, it's unbelievable," Bashir Ahmed told reporters.
The Times quoted police sources as saying Tanweer had once been arrested for disorderly behavior, and Hussain was questioned about shoplifting. The newspaper quoted sources as saying the name of one suspect "emerged" during an anti-terrorist investigation, but he was not arrested or questioned.
Sky News and The Associated Press contributed to this report.