-- Investigators in Britain and across Europe began examining whether the bombers who killed at least 49 people during London's morning rush hour Thursday were part of the same network of Islamic radicals blamed for the deadly train explosions in Madrid last year.

Spanish authorities said they were collaborating closely with British investigators and had dispatched a team of explosives experts to London who were responsible for the forensic analysis of the bombs that killed 191 people in the Madrid attacks on March 11, 2004.

British investigators said Friday the four bombs that exploded on three subway trains and a bus weighed less than 10 pounds each. In Madrid, the 10 bombs left in backpacks on commuter trains also exploded during the morning rush hour and weighed 22 pounds on average.

In both cases, timing devices were apparently used. Spanish officials said the Madrid plotters crafted the detonators with cell phones whose alarms were timed to go off together. British investigators are still trying to piece together evidence about the composition of the bombs in London. But Ian Blair, the London police chief, said there was "nothing to suggest there was a suicide bomber involved in this process."

"My guess is that the specialists have taken with them all the information obtained from the backpacks found here in Spain and will contrast them with what's there, to see if they come from the same place," said Rafael Vidal Delgado, an adviser for a private security firm in Madrid and a former colonel in the Spanish army. "Obviously, the same type of explosives would provide an indication of collaboration."

British law enforcement officials said they had made no arrests; they would not say whether they had developed any suspects. Investigators cautioned that they had not ruled out any groups that might have planned the attacks. Home Secretary Charles Clarke, who is in charge of domestic security, said the search for the bombers was like "looking for needles in a haystack." But he added that a claim of responsibility posted on the Internet by a group identifying itself as the Secret Organization of al Qaeda in Europe "is something we certainly take seriously."

In the Madrid attacks, Spanish investigators and prosecutors have charged that a network of Islamic extremists consisting mainly of Moroccan immigrants was responsible. Several defendants who have been indicted in the case are accused members of the Moroccan Islamic Combatant Group, which North African and European counterterrorism officials said has forged an alliance with al Qaeda but is an independently run network.

European law enforcement officials have broken up cells organized by the Moroccan network in the Netherlands, Italy, Belgium, Spain and France in the past 18 months and characterize it as one of the primary terrorist threats on the continent. Moroccan officials have blamed the group for orchestrating the May 2003 suicide bombings in Casablanca.

The network has also developed a presence in Britain in recent years. Mohammed Guerbouzi, a man who Moroccan officials charge is a founder of the group, has lived in London for years. Morocco issued an international warrant for his arrest two years ago, although British officials have said there was not enough evidence to take him into custody.

Investigators have long known that suspects in the Madrid bombings had contacts in Britain. One month after the attacks, seven suspected members of the Madrid cell blew themselves up as they were surrounded by police. Investigators later determined that the suspects had placed three phone calls to Britain shortly before killing themselves.

"The British are definitely comparing notes on Moroccan cells across Europe, in Italy, France and the U.K.," said Charles Powell, assistant director of the Elcano Royal Institute in Madrid. "The current thinking is that the people who did this may have trained in the same camps as the people who committed the Madrid bombings. The techniques are very similar."

Security analysts said the Moroccan Islamic Combatant Group and other North African groups in Britain have been especially active in raising money and producing false identity documents for radical Islamic causes.

Most other Islamic extremists in Britain are attached to Pakistani and South Asian networks, while Arab radicals from the Persian Gulf states represent a lesser threat, said Magnus Ranstorp, director of the Center for the Study of Terrorism and Political Violence at the University of St. Andrews in Scotland. While such networks sometimes overlap, he said, cell members usually belong to the same nationality or grew up in the same immigrant communities.

"I'm sure investigators are working both dimensions," Ranstorp said. "Is the center of gravity or focus on Pakistan and Bangladesh, or is the focus on the North Africans?"

A U.S. law enforcement official said yesterday that information gleaned from a suspected al Qaeda leader who goes by the nom de guerre Abu Faraj Libbi indicates that the group was interested in carrying out an attack similar to Madrid. Libbi, whose name means he comes from Libya, was captured in May in Pakistan. He is now in U.S. custody in an undisclosed location.

The U.S. official also cautioned that Libbi provided no firm details about such a plot, which suggests that it was only wishful thinking. Libbi also has told interrogators that while radical Islamic groups would ideally like to strike the United States, Europe provides an easier target because of less stringent security and border measures, the official said. "They're still interested in us, but we're a harder target right now," the official said.

Two U.S. intelligence officials, who spoke on condition of anonymity, said a connection between the Madrid network and the London bombers was one of several working theories being pursued by investigators. They said they were giving equal weight to the possibility that other groups were behind the attacks.

U.S. and European counterterrorism officials have also named Abu Musab Zarqawi, a Jordanian who is the leader of a network of foreign fighters in Iraq, as a possible inspiration for the London bombings. Staff writers Dan Eggen in Washington and Dafna Linzer in New York, staff researcher Julie Tate in Washington and special correspondent Jennifer Green in Madrid contributed to this report.

Police and rescue workers survey the damage done to a subway train. Bombs exploded on 3 subway trains and a bus.