LONDON — In a dramatic breakthrough in the largest manhunt in British history, police Wednesday stormed a house in Birmingham and fired a stun gun to immobilize and arrest one of the four suspected bombers in the second terrorist attack within a month on London's transportation system.

The arrest of Yasin Hassan Omar, 24, a Somalia-born refugee who arrived in England at the age of 12, occurred as investigators stepped up their search for at least three other would-be bombers in the botched attacks on July 21.

The search took on even greater urgency as signs emerged that the three suspects who remain at large may have more explosives and may try to strike again, particularly if police close in around them.

Acting on a tip from a neighbor of Omar, according to news reports, heavily armed police carried out a dawn raid on a house in the Small Heath area of Birmingham, 120 miles northwest of London, and used a Taser stun gun to capture Omar, said Peter Clarke, head of Britain's antiterrorism police.

Omar was apparently alone in the house at the time and was carrying a backpack when he was arrested.

Members of Britain's bomb squad, some dressed in armored suits, were pictured live on British television entering the home after police evacuated 100 nearby residences in a quiet, ethnically mixed neighborhood of Britain's second-largest city.

Omar was taken to London's high-security police station at Paddington Green. Police alleged that he was the same man pictured on closed circuit television fleeing the Warren Street Underground station in London on July 21 just moments after detonating, but failing to ignite, an explosive device in a purple backpack.

At another home in Birmingham, three more men — who police said were not suspected bombers — were arrested early Wednesday and held in Birmingham for questioning. No gunshots were fired, police said, in raids carried out by 50 officers from London's Metropolitan Police Anti-Terrorist Branch and West Midlands Police.

In a statement to reporters, Clarke said, "This of course is an important development in the investigation. However, I must stress how important it is for the public to remain watchful and alert. We are still looking for the other three suspects."

Clarke also released a new closed circuit television image of one of the other suspects, who police believe tried to detonate a bomb on a train near the Shepherd's Bush subway station in West London. In the photo he is wearing a white muscle shirt and standing on a bus he hopped on after eluding police.

It was another day of fast-paced developments in the investigation, which is now stretching across London and elsewhere in Britain.

In North London, police carried out more dawn raids in the neighborhoods of Finchley and Enfield. Forensic teams and explosives experts were investigating the residences, police said, but no arrests were made.

In South London, armed police Wednesday night raided a home in the ethnically mixed area of Stockwell, where Omar is believed to have entered the Underground carrying the backpack laden with explosives on July 21 before traveling toward Warren Street station where he attempted to detonate the device.

A police spokeswoman said three women from the Stockwell area were arrested on suspicion of harboring the July 21 suspects.

ABC News Wednesday night showed chilling photos of what it said were a number of nail bombs found in the trunk of a car parked by a member of the July 7 cell at a train station in Luton, on the outskirts of London.

The investigators are studying whether the explosive in those bombs is related to the material used in either the July 7 bombings or the failed July 21 attacks.

In a harrowing dragnet described by Metropolitan Police commander Sir Ian Blair as a "race against time," police have meticulously assembled key pieces of a forensic puzzle.

Those pieces range from the plastic food buckets used to contain the explosives to the suspected bomb factory in a storage garage at a public housing complex where police say Omar and another suspect, Muktar Said Ibrahim, 27, lived in Birmingham.

Police are taking seriously witness accounts that the two suspects returned to their apartment in the hours after the July 21 attacks, raising the fear that they may have picked up more explosives there.

Investigators also made significant strides in the last two days in developing a clearer picture of the terrorist cells through a web of connections stretching across ethnic lines within the Muslim community and some of London's melting-pot neighborhoods on the north and south sides.

Three of the July 7 bombers were of Pakistani descent from the northern city of Leeds; the fourth was of Jamaican origin who converted to Islam at a South London mosque.

The July 21 suspects included at least two Africans. Omar was born in Somalia and came to London as a refugee. Ibrahim, with whom he shared a ninth-floor apartment in a public housing complex in North London, came from Eritrea in 1992 as a child refugee.

Police have also gained significant ground in linking the two cells.

Police are investigating a white-water rafting trip in North Wales in early June which was attended by at least two of the July 7 bombers and possibly by several of the July 21 suspects.

Police believe the trip may have been a kind of team-building exercise for what they believe is a sophisticated network of Islamic militants inspired by the apocalyptic message of Osama bin Laden and expressed through his international network of holy warriors broadly known as al-Qaida, which is Arabic for "The Base."

After an explosive device was found on Saturday in a wooded area near the infamous Wormwood Scrubs prison in Northwest London, investigators said they believed that a fifth would-be bomber was involved who failed to detonate the device and discarded it as he fled.

In a sign of the frenetic pace of the investigation, commander Blair said his officers had responded to 250 suicide bomb scares since the July 7 attacks, and that on seven occasions police had been on the brink of executing what is commonly known as a "shoot to kill" policy.

Blair's comments came amid an intense debate in Britain over a controversial change in the rules of engagement in the age of suicide bombers, after police shot an innocent man from Brazil eight times at point-blank range, mistakenly believing that he was carrying a concealed bomb.

The man turned out to be Jean Charles de Menezes, 28, an electrician from Brazil. Menezes's body was being flown home to his native Brazil Wednesday for a funeral.