Experts said the attacks bore many signatures of the fragmented but virulent networks that have operational or ideological ties to Osama bin Laden's organization: multiple targets, near-simultaneity, significant civilian casualties and political timing.

But the lack of details about the blasts prompted debate among experts about whether the plot was the work of a longtime cell based in Britain, recently arrived operatives from Iraq, or a combination of the two.

The bombs went off as US President George W Bush sat down with Prime Minister Tony Blair and other world leaders at the Group of 8 summit in Scotland, a day after London was chosen to host the 2012 Summer Olympics.

The timing recalled the car bombings against British targets in Istanbul, Turkey, in 2003 as Bush met with Blair in London, as well as last year's bombings of commuter trains in Madrid that killed 191 people three days before Spanish national elections.

Some investigators suspect the plot involved Islamic extremists from Europe who went to Iraq, gained combat experience and ideological fervor, then returned to wage their holy war.

Before Thursday's attacks, investigators say, they had been concerned by the increasing presence in Europe of veterans of the Iraq conflict. During the last six months, Western intelligence reports described a ``redeployment'' on to the continent of operatives of Abu Musab al-Zarqawi, the suspected leader of al-Qaeda in Iraq.

A British law enforcement official said Britain's counter-terrorism agencies, which have been effective at infiltrating Islamic extremist groups at home, would find it more difficult to detect foreign fighters back from Iraq.

``I think it's more likely to be returnees, perhaps people connected to the Zarqawi group,'' said the British official. ``It doesn't feel like home-grown. We have got good contacts in the Muslim community, and you would have thought at some point someone would have detected something. Unless it was a group that completely slipped in under the wire.''

LOS ANGELES TIMES