Police in Britain arrested 12 men Monday in early-morning raids that they said were necessary to head off a potential terrorist attack.
John Yates, assistant commissioner of Scotland Yard, declined to give details of any alleged plot but said the men were detained "on suspicion of the commission, preparation or instigation of an act of terrorism" in Britain.
It was the largest such sweep in the country since April 2009, when authorities across northern England arrested a dozen men for alleged involvement in what officials called a "very big" and imminent terrorist attack. All 12 of those suspects were released two weeks later for lack of evidence, an embarrassing turn for law enforcement.
Yates said Monday that the new sweep was unavoidable, carefully planned and based on actionable intelligence.
"With the information we had, I believe today's arrests were absolutely necessary in order to keep the public safe," he said.
The suspects, who range in age from 17 to 28, were, with one exception, arrested about 5 a.m. at or near their homes in London, in the northern English city of Stoke-on-Trent and in Cardiff, the capital of Wales. One man was arrested away from home in Birmingham, in central England.
The predawn sweep involved the cooperation of at least four law enforcement agencies and a commitment of "significant resources," Yates said. The officers were unarmed during the raids, suggesting that any plot by the suspects was not in its advanced stages.
British news outlets said several of the men were from Bangladesh but that the others were British, a report that, if the case against them holds, is likely to stoke the debate in Britain over the threat of homegrown terrorism. The country has agonized over the issue since 2005, when suicide attackers who bombed the London public transit system, killing 52 people, turned out to be native Britons.
Citing unnamed intelligence sources, news reports also said the 12 men arrested Monday may have intended to strike multiple high-profile targets, including the government quarter in downtown London. However, the suspects did not appear connected to an attempted suicide bombing Dec. 11 in Stockholm, the Swedish capital. That attack killed only the assailant, an Iraqi-born Swede who maintained a home in the city of Luton, outside London.
Investigators declined to say whether the alleged plot in Britain was related to a statement by an Iraqi official last week that Al Qaeda was planning attacks on the U.S. and Europe during the Christmas season. At this time last year, a Nigerian man was caught with explosives in his underwear aboard a transatlantic flight in what authorities said was an attempted bombing inspired by Al Qaeda.
Authorities said that searches of the 12 suspects' homes could take days.
Under Britain's anti-terrorism laws, the men can be held for questioning without bail for up to four weeks.