The London terror bombings were the first suicide attacks on British soil and were carried out by homegrown terrorists.

One of the four suicide bombers was thought to be Shahzad Tanweer, aged 22, who friends described as a cricket-loving sports science graduate, and another of the bombers was only a teenager.

At least three of them are believed to have been British males of Pakistani origin who lived in West Yorkshire. Detectives are still unsure about the identity of the mystery fourth bomber.

A relative of one of the suspects was arrested in West Yorkshire yesterday and was being brought to London to be quizzed by the antiterrorist branch.

Just 20 minutes before blowing themselves up, killing about 50 people, the four suicide bombers were captured on CCTV at King's Cross station, talking casually as they carried their devices in armystyle rucksacks.

A senior security source, who has viewed the footage, said: "They were chatting. You would think they were going on a hiking holiday."

The images, which were only spotted by detectives at 8pm on Monday, provided a crucial breakthrough in the police investigation which saw dramatic developments yesterday, beginning with a series of dawn raids in West Yorkshire.

Bomb-making equipment was found at one of the six addresses raided in Leeds and Dewsbury.

Later, police also found explosives in a car parked at Luton station - where the four bombers are thought to have met up - and a series of controlled explosions were carried out. Detectives and the security services now fear there could still be a second suicide bomb team waiting to strike.