After a day of intensive police activity, detectives say at least one of four suspected London bombers died in last week's Tube and bus blasts.
Security sources said it was likely three men whose belongings were found at the scenes are dead - there is a question mark about the fourth bomber.
Explosives were found in Leeds and Luton after a series of dawn raids.
The BBC's Frank Gardner said it was unlikely the British men - who have been spotted on CCTV - acted alone.
Police said they had arrested a relative of one of the four suspects in Yorkshire and taken them to London for questioning.
Four bombs - three on the London Underground and one on a bus in Tavistock Square in the city centre - killed at least 52 people on Thursday.
Sir Iqbal Sacranie of the Muslim Council of Britain said they had received the latest news from the police with "anguish, shock and horror".