07/12/2005 at 03:00 PM EDT
Six warrants have been issued and four men pinpointed as part of the complex and intensive investigation into Thursday's bomb attacks in London, Scotland Yard announced.
Police confirm that one man has been arrested and is in custody.
Four of the suspects in the West Yorkshire area of England are believed to have been suicide bombers – three in the city's subway system and another on a double-decker bus, setting off their explosives almost simultaneously.
Police had "strong forensic and other evidence" that the man who carried a bomb onto a subway train that exploded between the Aldgate and Liverpool Street stations was killed in the blast, said Peter Clarke, head of the Metropolitan Police anti-terrorist branch, in a news conference. Confirmation from the coroner is being awaited.
Another suspect was reported missing by his family at 10 a.m. Thursday, and some of his property was found on the bus in which 13 died, Clarke said.
Closed-circuit TV video showed the four arriving at King's Cross station by 8:30 a.m. on Thursday, some 20 minutes before the blasts began, he told reporters.
The official death toll from Thursday now stands at 52, though that number is likely to climb as more bodies are retrieved from the Underground.
Meanwhile, in Leeds, about 185 miles north of London, British soldiers have reportedly searched residences in the area for additional explosives and computers. Police said materials found in their investigations could be harmful to the public, but provided no further details and asked people to remain vigilant.
London police called its series of raids part of a "pre-planned, intelligence-led operation."
Police also shut down a rail station in Luton, 30 miles north of London, as they removed a controlled explosive on a car they suspect is linked to the terrorist attacks, according to the British Press Association.
Back in London, Prime Minister Tony Blair went to City Hall to sign a book of condolence for the victims. He wrote: "With deep condolences for all those who lost their lives and for their families who mourn and with heartfelt admiration for London, the greatest capital city in the world."