London police released surveillance photos Friday of four men suspected of trying to bomb the city's transportation system Thursday, and antiterrorist officers fatally shot a man they were pursuing at a subway station in connection with the attack.

Police later arrested a man in south London's Stockwell neighborhood, the same area where the shooting occurred. A second man was arrested at a train station in Birmingham but was subsequently freed without charge after a search of two suitcases turned up nothing incriminating. No information on the men's identities was immediately released. The station, which had been shut down while explosives experts searched the luggage, was reopened.

It was not clear whether the man arrested in Stockwell was among the four suspected bombers being sought or otherwise connected with the plot. Police also did not identify the man who was shot at the Stockwell subway station Friday morning, but officials said the dead man was not one of the four whose photos were released.

In a news briefing, top officials of Scotland Yard appealed for the public's help in identifying the four men, who were shown on surveillance video cameras allegedly leaving the scenes of attempts to bomb three subway trains and a double-decker bus in different parts of London around midday Thursday.

As the officials spoke, police were carrying out raids at three London addresses as part of a fast-moving investigation of the latest terrorist attack on the city's subway and bus system.

Unlike a devastating attack two weeks earlier that killed 56 people -- including four suicide bombers -- on three subway trains and a bus, the attacks Thursday failed because the homemade explosives carried by the would-be bombers did not fully detonate at each of the four sites, police said. As a result, only one person was reported injured.

London's police commissioner, Ian Blair, said the shooting of a man at the Stockwell subway station at about 10 a.m. Friday was "directly linked to the ongoing and expanding antiterrorist operation" aimed at finding those responsible for the latest attacks.

"The man was challenged and refused to obey police instructions," Blair said at the late afternoon news briefing. He did not elaborate on the shooting or explain how the man was allegedly linked to Thursday's attempted bombings.

The incident came amid an apparently widening law enforcement manhunt involving searches at a number of locations.

Witnesses told reporters the man was pursued by several armed plainclothes officers at the Stockwell station, which is just one stop from the Oval station where one of the attempted bombings occurred Thursday. The man vaulted over barriers as he was being chased, eventually running into a subway car, where he was shot several times, witnesses said.

Witnesses told BBC radio that they heard police shout, "Get down! Get down!"

The man "half-tripped as he ran," one witness reported, and was on the floor of a waiting train as shots rang out.

"We can confirm that just after 10 a.m. armed officers entered Stockwell Tube station," said a police statement. "A man was challenged by officers and subsequently shot. London Ambulance Service attended the scene. He was pronounced dead at the scene."

A witness told BBC radio that "there was a lot of shouting and then gunfire" at the station at about 10 a.m. local time (5 a.m. EDT). The dead man was described as wearing a baseball cap and a thick coat -- unusual for the warm summer weather and possibly a source of suspicion that he was concealing something on his body. Several witnesses interviewed on the radio said he was "Asian," a term that in Britain includes people of Pakistani or other South Asian origin.

Service was suspended on the Underground subway system in the area. The Stockwell station is a junction stop south of the River Thames on the Victoria Line and the Northern Line on the Underground system.

In the news briefing, Blair said police "are facing previously unknown threats and great danger." He appealed for "understanding and calm" and assured the public that the police operation "is targeted against criminals" and not any minority community or section of a community.

The London police assistant commissioner, Andy Hayman, said police have obtained closed circuit television images of four men whom authorities "urgently want to trace" in connection with attempts to detonate four bombs Thursday.

The first picture was of a man wearing what appeared to be a dark sweatshirt with the words, "New York," written in white across the front. Hayman said the man apparently left an explosive device in a subway train at the Oval station and was running away at about 12:34 p.m.

The second man left a device on the top deck of a Route 26 double-decker bus about half an hour later, Hayman said. He was wearing a gray T-shirt with a palm tree design, a dark jacket and a white baseball cap.

The third suspect, a dark-skinned man in dark clothing, left a device on a Victoria Line subway train at the Warren Street station and was shown on a surveillance camera leaving the station at 12:39 p.m.

The fourth man was carrying a backpack as he ran from the Shepherd's Bush subway station at 12:21 p.m. wearing a dark shirt and trousers and a dark baseball cap. He was later reported to be wearing a white vest, Hayman said.

He said that "bombs partially detonated at each of the four sites" and that they were homemade explosives in dark-colored bags or rucksacks. He said it was too early in the investigation to tell how the devices were detonated.

Hayman and Blair asked members of the public to call police if they spot any of the men, know who they are or can recall anything about their movements. They asked people not to approach the men.

As the manhunt continued, police cordoned off an Internet cafe on Harrow Road in West London, where they searched a building, the BBC reported. Police said no arrests have been made at that location.

Barbash and Branigin reported from Washington.