They urged witnesses to come forward and help with an investigation that appears to be on the trail of a possible mastermind for last week's deadly attacks.
As Britons gathered to remember the bomb victims, police raised the confirmed death toll from the attacks to 54, including the four suspects. Unreleased photographs show the four men, each carrying a hiking-style backpack, meeting inside the King's Cross station shortly before three bombs exploded on subway trains about 8:50 a.m., police said; the bomb on the bus exploded 57 minutes later. Press reports in Britain have said those photographs were not released because they also show a fifth suspect, whom police are still trying to locate.
The blast aboard the No. 30 bus offered investigators many of their early clues, including a decapitated head -- considered by bombing experts a telltale sign of a possible suicide attack, British news media reported.
But the blast also paints a confusing picture of the attackers' actions and motives. Some police officials speculate that Hussain intended to board one of the city's northbound subway trains -- part of a plan to stage attacks at all points of the compass -- but was confounded by interruptions on the Northern line's service.
Police said 80 passengers were aboard the bus when the bomb exploded, killing 13, and survivors have told of hearing a ghastly scream in the second before detonation.
Peter Clarke, head of Scotland Yard's anti-terrorist branch, said investigators are trying to determine Hussain's movements between the time he was photographed at King's Cross and the time of the bus blast, 81 minutes.
"Was he alone or with others?" Clarke asked, pleading for witnesses to call police. "Do you know the route he took from the station? Did you see him get on to a No. 30 bus? And if you did, where and when was that?"
Throughout the United Kingdom yesterday, residents observed a noontime moment of silence to honor the blasts' victims, bringing cities, streets, airports and the British Open golf championship to a two-minutes standstill. In the evening, officials held a vigil in London's Trafalgar Square, during which Mayor Ken Livingstone spoke with tears falling from his face about the contrasts between the attack and the announcement, a day earlier, that London will host the 2012 Olympic Games.
"In seven years' time, when the games begin, sitting at the front of the stadium, and watching the 200 teams that will come from every nation, will be those who were maimed but survived, and the relatives of those who died," Livingstone said.
"Those who came here to kill last Thursday had many goals, but one was that we should turn on each other, like animals trapped in a cage. And they failed, totally and utterly."
The investigation continued throughout England yesterday, including in the northern city of Leeds where police were searching three homes linked to the suspects. The Times of London reported that police found an apparent "bomb factory" at one of the raided houses, and police said they found suspected bomb materials in a car the suspects are thought to have parked at the Luton train station.
Aside from Hussain, authorities have identified Shahzad Tanweer, 22, as one of the bombers. News reports, citing police sources, identified another suspect as Mohammad Sidique Khan, 30. All three were believed to have been from Leeds.
Police have said that personal documents bearing three of the suspects' names were found in the blast wreckage. They have not identified the fourth suspect, but the BBC reported yesterday that they are focusing on a Jamaica-born Briton who lived in Aylesbury, outside London. Police raided a house in Aylesbury on Wednesday evening.
The Times and other news media have also reported that police are searching for a 33-year-old Egyptian who recently taught or studied chemistry at the University of Leeds. The man is believed to have rented one of the homes in Leeds that police are searching.
The Associated Press reported that FBI agents in Raleigh, N.C., joined the search for the man, identified as Magdy Asi el-Nashar, who was believed to have been a former North Carolina State University graduate student.
Some Leeds residents who were evacuated from homes near the raids were waiting to return yesterday morning, saying they are thankful the investigation is progressing but are eager for it to be over.
"It's not a proud time for West Yorkshire," said Kevin Caldwell, who lives near Leeds' Hyde Park neighborhood, referring to the area in England where the city is located. More than 600 people were evacuated from homes in Hyde Park, where police believe the suspects manufactured or assembled their explosives.
England has one of the most comprehensive electronic surveillance networks in the world, including a web of cameras that is believed to photograph the average Londoner several hundred times a day. Besides surveillance, the system is also used to spot vehicles violating London's anti-congestion statutes. The subway system alone has roughly 6,000 cameras, according to city officials.
Livingstone has said since the attacks that he plans to increase coverage until "there will hardly be a park or transport system where we haven't put it."