Friday, July 22 2005 17:31 Hrs (IST) - World Time -

London: A massive hunt has been launched to trace the four would-be bombers who attempted to bomb three Underground stations and a bus in London as detectives scoured the sites for fingerprints, forensic evidence and other clues.

Police said two persons were arrested in connection with the explosions, including one from the Downing Street area, which has the residence of Prime Minister Tony Blair.

But both were released without any charge after interrogation, they said.

Metropolitan Police Chief Sir Ian Blair said forensic material gathered from the sites of the blasts could prove a 'significant break' in solving the incidents.

Police were focussing on forensic evidence from the scene as well as CCTV footage, he said.

The explosions at Warren Street tube station in central London, Oval in south London, Shepherd's Bush in west London and on a No 26 bus at the junction of Hackney Road and Columbia Road in Hackney, east London were 'pretty close to simultaneous', he said.

Several witnesses had come forward with first hand accounts of events and descriptions of the bombers.

Britain's Press Association news agency quoted unidentified sources as saying detectives were working on the belief that the bombs were not properly primed a factor that limited the damage.

Four explosions rocked three tube stations and a bus here at midday yesterday (July 21, 2005) bringing back grim memories of the July 7 terror attack that killed at least 56 people.