The fourth bomber who caused maximum damage in the King's Cross station was identified by neighbours in Leeds as Ejaz or Nadim Fiaz, a man in his early 30s.
Fiaz, known to his friends as Jacksy, is believed to have travelled separately to Luton where he met the three other bombers, The Times daily reported on Thursday.
Police had on Wednesday identified three bombers as Hasib Hussain (19), Shehzad Tanweer (22), and Mohammed Saddique Khan (30), all from West Yorkshire in northern England.
A house in the Beeston area of Leeds, where Fiaz's family used to live, was cloaked from view on Wednesday by a blue tarpaulin behind which police were conducting a search.
Neighbours said the house, which has been empty for several years since Fiaz's parents separated, was a scene of regular, late night visits.
"Young men would turn up at the house at 2 am and 3 am and hold meetings that would last for hours," a neighbour said, adding that these meetings were going on for a long time.
Detectives are trying to establish who may have attended the meetings and if they were part of the terrorist cell that planned the London bombings which killed 52 people.
Fiaz's brother, Naveed, has been identified locally as the man being questioned by anti-terrorist branch detectives at Paddington Green police station, the report said.
His home, near the former family residence, became the seventh property in West Yorkshire to be sealed off in the widening operation.
According to the report, Naveed, 29, whose English-born wife is expecting their fourth child, handed himself in to police voluntarily for questioning.