British anti-terror police have raided several houses in an around the northern English city of Leeds, in connection with the bombing in London last week.
The raids began early Tuesday in a combined action by counter-terrorist police and the army.
Several hours into the operation, authorities evacuated several hundred residents in the Burley section of Leeds to allow soldiers to safely blast their way into a vacant house.
A police spokesman said authorities were looking for explosives like those used in the London bombings that killed more than 50 people and wounded 700 others.
London Police Commissioner Ian Blair says Tuesday's raids are an important development.
"They are an intelligence-led operation run by the Metropolitan Police Service in our anti-terror role supported by West Yorkshire police," he said. "We think these are significant."
Mr. Blair also expresses confidence that the perpetrators of the London attacks will be captured.
"The Metropolitan Police Service has a long and proud tradition in which we have caught almost every bomber that has ever dared to carry out atrocities in London, and we are implacably resolved to do this," he said.
British police sources say Tuesday's raids support a theory that the bombers may have taken a two-hour train journey from Leeds to London's King Cross station, and from there easily dispersed to plant explosives that blew up three subway trains and a city bus during rush hour last Thursday.
British Prime Minister Tony Blair says Islamic terrorists probably carried out the attacks and Leeds has one of Britain's largest Muslim communities.