Flowers are placed at entrance of King's Cross Underground Station which was hit by explosion, Thursday

London police say three of the bombs that killed 49 people and wounded hundreds more were detonated on subway carriages within one minute of each other, indicating they may have been set off by timing devices, and not suicide bombers.

Police say the latest evidence points to a closely coordinated attack by the bombers on London's subway system.

Deputy Assistant Police Commander Brian Paddock told a news conference a review of data reveals the bombs on three subway trains exploded within seconds of each other, rather than over the course of 26 minutes, as originally believed.

Mr. Paddock indicated the timing could mean the bombers escaped, and are still at large.

"Either you have people with the explosive devices, who have synchronized watches or whatever, and they have all simultaneously detonated their devices at the same time, or it could be that these devices have been triggered by timing devices," he explained. "Bearing in mind these were almost simultaneous, we think within 50 seconds of each other, maybe that lends more towards timing devices, more than people with the bombs manually detonating."

Remnants of bus that exploded in Tavistock Square, in central London, Thursday

Police are also investigating a fourth blast that ripped through a double-decker bus about an hour after the subway attacks.

In other developments, police said crews have not yet retrieved all the bodies from a subway carriage trapped deep underground on the Piccadilly Line, between the King's Cross and Russell Square stations.

Police say the work could last for days, because of the difficult conditions, including heat, dust and concern the tunnel might collapse.

As the recovery work goes on, attention has turned to people still missing and feared dead. Friends and relatives of missing persons have made desperate appeals for information in newspapers and on broadcast outlets.

Authorities suspect Islamic militants linked to or inspired by al-Qaida could have carried out the bombings. Two obscure groups have claimed responsibility in the name of al-Qaida.

Britain's mainstream Muslim organizations have denounced the attacks, and, despite fears of backlash, authorities say there have been no serious incidents of retaliation against British Muslims.