LONDON – Police commandos firing tear gas and stun grenades on Friday seized two suspected bombers in London, and authorities in Rome arrested a third, in a dramatic conclusion of the hunt for the four men wanted in last week’s failed attacks on London’s transit system.

Police officials, who already had one man in custody and feared the others would strike again, were jubilant over the breakthrough. The arrests appear to give them a rare prize: a full cell of alleged terrorists captured alive and unharmed, who could be questioned about possible links between their abortive attacks and the July 7 bombings that killed at least 56 people, including four presumed bombers.

In a day of fast-moving developments, police arrested another man in London’s fashionable Notting Hill neighborhood and are investigating whether he was responsible for an unexploded fifth bomb that was found abandoned in a park.

Police remain uncertain as to who else might have aided in the plot. Peter Clarke, head of the counterterrorism branch of London’s Metropolitan Police, maintained a cautionary tone after the arrests. “We must not be complacent,” he warned in a televised statement. “The threat remains, and is very real. The public must be watchful and alert.”

The four suspects are alleged to have carried explosives onto three trains and a bus on July 21 in what investigators called an attempt to replicate the carnage of July 7. But all four bombs failed to explode, and the men fled the scene.

An enormous manhunt ensued, with police blanketing Britain with security camera images of the suspects. One of them, Yasin Hassan Omar, 24, was captured Wednesday in Birmingham. He is accused of trying to blow up a subway train near the Warren Street station.

Besides the four suspects, the authorities are holding 20 people for questioning, some of whom are suspected of having helped the alleged bombers to hide in the days after the July 21 attacks.

Meanwhile, the Brazilian man gunned down in a subway car July 22 by police who mistook him for a suicide bomber was buried before thousands of people Friday evening in Gonzaga, Brazil, his birthplace.