Police traced three of the four bombers back to West Yorkshire.
They were first alerted when the mother of Hasib Hussain, 18, reported him missing after the blasts. He had told his family he was going on a day trip to London and had not been heard of since.
Officers found his property on the bombed bus at Tavistock Square and began to link him with three other men at the other bomb locations.
On Tuesday 12 July, police carried out raids on six houses in Leeds and Dewsbury.
They searched Hussain's family home on Colenso Mount in the Holbeck area. In neighbouring Beeston they also raided the Colwyn Road home of his friend Shehzad Tanweer, 22, and a further unoccupied house on Stratford Street.
Meanwhile, officers sealed off an area around Alexandra Grove in the Burley area of Leeds and evacuated about 500 people after finding substantial quantities of explosives there.
In Dewsbury they raided addresses at Thornhill Park Avenue and Lees Holm, belonging to Mohammed Sadique Khan and members of his family.
Police also raided house in Aylesbury, Buckinghamshire, thought to be the home of the fourth bomber, Germaine Lindsay.