As police hunt for the London bombings mastermind, investigations have revealed that a national terrorist network may be behind the last week's serial blasts.
The discovery that the fourth bomber, a Jamaican-born Briton, came from a different background than the other three, Britons of Pakistani origin, suggests that the bombers were part of a wider network.
Complete Coverage: The London Blasts
Intelligence officers are trying to establish links between the two groups, whether they were both in contact with a radical cleric or had met in Pakistan.
After police released CCTV images of Hasib Hussain, the man who blew up the bus at Luton station at 7.20 am on July 7, anti-terrorist officers and Mi5 are trying to track down other members of the terrorist unit.
They are thought to include a bomb-maker and a planner. Among those being investigated is a man in his mid-40s who
stayed at the Leeds house where the bombs were made, but left the country for Iraq months ago.
Police are also hunting for an Egyptian chemistry student who disappeared from his Leeds home.
The three bombers from Leeds -- Mohammad Sidique Khan, 30, Shahzad Tanweer, 22 and Hasib Hussain, 18 -- were friends, all British-born of Pakistani origin.
As part of the Leeds inquiry, army bomb-disposal experts are examining a former community centre in the Beeston area of Leeds.
Reports said explosives were found in a Leeds home which were similar to those used in the Al Qaeda linked attacks.
"It is in fact acetone peroxide...it is the same kind of explosive Richard Reid had in his shoes when he tried to blow up a trans Atlantic flight in 2001," BBC said.