LONDON - Police made nine arrests on Thursday in their hunt for the men behind failed attacks on London's transport system on July 21 and warned Britons that more cells of would-be bombers could strike.
Three weeks ago another team of suicide bombers killed 52 people in the capital. Police say four British Muslims carried out those bombings, which they have linked to al Qaeda.
London police chief Ian Blair said the capital had been lucky that bombs on three underground trains and a bus had not exploded fully a week ago. Three men still wanted for those attacks remained a danger and might not be the only threat.
"This is a campaign we are facing, it is not a one-off event," Blair told a police authority meeting.
"It does remain possible that those at large will strike again. It does also remain possible that there are other cells that are capable and intent on striking again," he said.
The two waves of attacks have put London on high alert, with police maintaining a high profile around the city.
"This is not the B-team, these weren't the amateurs," Blair said of the second group of attackers. "They made a mistake -- they only made one mistake and we're very, very lucky."
No one was hurt in last week's attacks.
Police arrested nine men in Tooting, south London, early on Thursday, bringing to 20 the number of people being held in connection with the failed July 21 attacks. (Please see Nine More Arrested in London Bombing Investigation)
Police said the nine did not include the three suspected bombers they are still hunting. They used a stun gun to arrest one of the prime suspects, Yasin Hassan Omar, in a dawn raid in the city of Birmingham on Wednesday.
Omar, 24, came to Britain from Somalia as a child refugee. He was wanted in connection with an attempted attack at London's Warren Street underground station on July 21.
Police Exhausted
Blair said police were reviewing 15,000 closed circuit television tapes, had taken 1,800 witness statements and received 5,000 calls on their anti-terrorism hotline.
He said his force was determined to catch the bombers but its biggest operational challenge was taking a heavy toll.
"I do ask for your understanding around the level of exhaustion now sitting in the metropolitan police service," he told the authority.
"I am looking at some very tired men and women."
Police swarmed across London, where residents have become used to the wail of sirens in recent weeks as members of the public report abandoned packages or people acting suspiciously.
Officers, some brought in from outside London, patrolled the streets outside stations, an unusual sight for commuters.
British Transport Police said some leave had been cancelled as the force maintained a high-profile campaign.
"We are on very high alert. It's part of a continuing effort to have high-visibility policing on stations," a spokesman said.
Opinion polls show a majority of Britons fear Islamist militants could wage a sustained campaign against their country.
Newspapers published front-page pictures of a ready-made nail bomb found in the boot of a car that had been rented by one of the July 7 attackers. They showed a bottle studded with nails to act as shrapnel.
But a police spokeswoman denied a report there were 16 ready-made bombs in the car. Apart from the nail bomb, investigators found bits of explosives and other components.
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