LONDON, England (CNN) -- Police have released photos of four men they want to trace in connection with Thursday's attempted bomb attacks on London's transport system.
The appeal for information about the bomb suspects came hours after the killing of a man by officers on Friday morning that police said was directly linked to the anti-terrorist operation.
Police said the man was shot as he tried to enter a train Friday morning at the Stockwell Underground station in south London.
"The information I have available is that this shooting is directly linked to the ongoing and expanding anti-terrorist operation," said Metropolitan Police Commissioner Ian Blair.
While the death is deeply regrettable, Blair said, the man challenged police and refused to obey instructions.
A senior police official told CNN the man was being followed in connection with the investigation. The man has not been formally identified, the official said, and it is not clear if he was one of the four suspected bombers police have identified from closed circuit television images.
The first CCTV image released by police Friday showed a young man in a dark top with "New York" written on it apparently fleeing after leaving a bomb on a train at The Oval station in south London.
The second image showed a middle-aged man with a moustache wearing a gray T-shirt with a palm tree on it standing on the top deck of the number 26 bus in Hackney, east London.
The third image shows a man leaving Warren Street Underground station in central London at about 12:39 p.m. on Thursday. He was wearing dark clothes.
The fourth image showed a man at Westbourne Park Underground at 12:21 p.m. He later traveled west on the Hammersmith and City line to Shepherds Bush underground where he ran off. He was wearing a dark shirt and trousers, and was later wearing a white vest.
Assistant Police Commissioner Andy Hayman said four homemade bombs in backpacks "partially detonated" on three subway trains and a double-decker bus in London Thursday. It is too early to tell how the bombs detonated, he said.
Meanwhile police were searching at least three locations in London as part of their investigation, Hayman said.
Blair said the officers in the capital are "facing previously unknown threats and great danger."
One commuter Teri Godly described how she had stood next to the man the British media described as a suspected suicide bomber before police charged in and shot him several times.
"A tall Asian guy, shaved head, slight beard, with a rucksack got in front of me," she told Sky News television.
"Shortly after that, as I was about to get onto the train, eight or nine undercover police with walkie talkies and handguns started screaming at everyone to 'get out, get out.'" (More eyewitness)
The incident just after 10 a.m. (0900 GMT) Friday triggered new fears about the security of the city's transit system.
The Stockwell station is one stop down and about a mile away from the Oval station, the scene of one of four attempted bombings on Thursday.
The London Underground said there were line suspensions on the Victoria and Northern lines, but later said the Victoria Line had reopened except for Stockwell station. Trains were passing through Stockwell but not stopping. The Northern Line was still suspended between Kennington and Morden stations.
The road passing by the Stockwell station was also closed.
In addition, authorities said one other station -- Highbury and Islington toward the north end of the Victoria Line -- was being evacuated after the incident at Stockwell, but they did not have further details.
The shooting is a rarity in London, where police generally are not armed except for special response units.
Police later said they were searching an address at Harrow Road, to the west of the city.
In addition, an east London mosque on Whitechapel Road said it had received a bomb threat and called police. The mosque was evacuated while police checked the building. It was cleared and people were allowed back inside.
The shooting Friday came as police were hunting the bombers who struck London's transport network the day before.
The would-be terrorists attempted to set off another string of bombs on London Underground trains and a double-decker bus Thursday, leaving behind "significant" forensic evidence when the devices failed to detonate, the city's police commissioner said.
"Our lucky day," said a banner headline in the Daily Mirror newspaper. "Four bombs, three trains, one bus, zero deaths."
The Sun newspaper ran the banner headline: "Four suicide bombers on loose."
The aftermath resulted in some Friday morning headaches for commuters as London transport officials kept the Warren Street and the Oval stations closed.
The incidents came two weeks after a series of blasts killed 56 people, including four men identified as the bombers, on another three trains and a bus.
But police Commissioner Blair said it was "too early to say" whether there was a connection between the events.
"Clearly, the intention must have been to kill. You don't do this with any other intention," Blair said. But he added: "The intention of the terrorists has not been fulfilled."
Police hotly defended their safety measures amid questions over any evidence suggesting a second round of attacks was likely.
Michael Bowron, assistant police commissioner for the City of London Police, told CNN: "We're making this place as safe as we can in the circumstances, and I think it's wholly unfair to talk about intelligence failures.
"Clearly, there's a new face to terrorism, and we're working very closely with our colleagues to find out what that nature is, and how to get into it, and how to prevent it in the future."
Intelligence expert Crispin Black told CNN: "Whatever happened yesterday, I suspect wasn't what the terrorists wanted to happen. It would appear that they are under some sort of pressure. It might be to do with their bombs -- people are asking 'where are the bombing masterminds?'
"And there is another plus point to this: We have got four people on the run, and the police appear to have good forensic evidence to build up a picture from.
"My guess is that all four of these people will not be able to get away. They have no escape plan if they were suicide bombers, and so the advantage is now playing to the security and intelligence agencies."
Witnesses reported small explosions aboard Underground trains north, south and west of the city center and aboard a bus in east London early Thursday afternoon. One person was reported wounded.
British Prime Minister Tony Blair said the incidents were designed "to scare people and frighten them."
"I think we just have got to react calmly and continue with our business as much as possible as normal," he added.
Witnesses' accounts led to suggestions that detonators had gone off but failed to trigger bombs. The police commissioner would say only that some devices "remain unexploded," and he said police evidence technicians were going over the scenes. (More on Thursday's blasts)
"We do believe that this may represent -- may represent -- a significant breakthrough, in the sense that there is obviously forensic material at these scenes which may be very helpful to us," Blair said. "So I feel very positive about some of these developments."
But he said the investigation was still in an early stage and cautioned against the "enormous amount of speculation" concerning the incidents. Police urged anyone with photos or video taken around the time of the incidents to e-mail them to investigators.
Police took two men into custody after the blasts, including one man arrested near the prime minister's residence at 10 Downing Street. He was released without charge early Friday, while the first man, who was arrested two stops away from the Warren Street station, was released a few hours earlier.
