LONDON – Four suicide bombers, at least three of them British, were responsible for the explosions that killed more than 50 people in London last Thursday, senior security sources said last night.

The three were all thought to be of Pakistani ethnic origin and said by neighbors to have lived modest suburban lives in West Yorkshire.

One has yet to be identified, while the others were not known to police or the intelligence services as terrorist suspects.

One was the son of a fish-and-chips shop owner. Another was only 18 years old and was reported missing by his worried parents at 10 p.m. on the day of the bombings.

Police said the bombers made a rendezvous somewhere outside London last Thursday morning, traveled together to King’s Cross-Thames link station wearing large military-style rucksacks containing 10 pounds of high-explosive bombs, and split up to attack their assigned targets.

Three of the attackers were said to be from the Leeds area and were identified locally as British-born men of Pakistani ethnic origin.

The revelations realized the worst fears of police and the Muslim community with confirmation of Western Europe’s first suicide bombers and of the first Britons to attack their own countrymen with explosives since peace in Northern Ireland.

Early yesterday, police raided six addresses in West Yorkshire, discovered what is thought to be a bomb factory, and arrested a man.

Three houses were the homes of three of the four men police believe were responsible for bombs in tube trains near Liverpool Street, Edgware Road, and King’s Cross underground stations at 8:50 a.m. last Thursday and on a bus in Tavistock Square almost an hour later.

Neighbors identified the bomber reported missing by his family, who police say died on the bus, as Hasib Hussain, from Holbeck, Leeds.

Detectives were examining the possibility that he either panicked or changed his mind about the suicide mission and carried his bomb onto the bus. It might have gone off accidentally or he could have set it off, perhaps after being challenged by passengers.

The Liverpool Street bomber is believed to have been Shehzad Tanweer, 22, the son of Mohammed Mumtaz Tanweer, a fish-and-chip-shop owner in Beeston, Leeds.

Friends said Shehzad Tanweer was a keen cricketer and “a good Muslim” and expressed disbelief that he could have been involved in England’s worst terrorist attack.

The disclosure that the bombs were the work of domestic rather than foreign extremists came from the Metropolitan Police deputy assistant commissioner, Peter Clarke.

He said: “We have identified CCTV footage showing the four men at King’s Cross shortly before 8:30 a.m.

“One, who set out from West Yorkshire was reported missing by his family shortly after 10 p.m. on Thursday. We have now established that he was joined on his journey to London by three other men.

“We have since found personal documents bearing the names of three of those four men close to the seats of three of the explosions.

“As regards the man who was reported missing, some of his property was found on the Route 30 bus in Tavistock Square.”

Survivors of that blast said they saw a man with olive skin searching agitatedly through a rucksack moments before a bomb went off.

Mr. Clarke said: “Property in the name of a second man was found at the scene of the Aldgate bomb.

“In relation to a third man, property in his name was found at the scene of both the Aldgate and the Edgware Road bombs. We also have very strong forensic and other evidence that it is very likely one of the men from West Yorkshire died in the explosion at Liverpool Street.”

Sources said the police were almost certain that scientific evidence would confirm that all the men seen on CCTV at King’s Cross died in the separate blasts.

The investigation seems likely to increase tension in multiracial communities, especially in West Yorkshire.

Calling for calm, the assistant commissioner of Scotland Yard, Andy Hayman, said: “No one should be in any doubt the work last Thursday is that of extremists and criminals.

“So, that being the case, no one should smear or stigmatize any community with these acts.”

Sir Ian said yesterday’s operation, involving armed police and Army bomb disposal specialists, was intelligence based and was “directly connected” to last week’s blasts.

A controlled explosion was carried out near Leeds Grand Mosque, houses were searched in the nearby town of Dewsbury, and police evacuated Luton railway station, carrying out explosions to recover a vehicle possibly linked to the attacks.