LONDON British investigators believe that the bombs used in the coordinated terrorist attacks here contained "military quality" high-grade explosives, British and European counterterrorism officials said.
Investigators said they still did not know whether the explosives contained plastic materials, or were made some other way. But they said the material used in the bombs was similar to the kind manufactured for military use or made for highly technical commercial purposes, such as dynamite used for precision explosions to demolish buildings or in mining.
Because of the small size of the bombs - estimated at 4.5 kilograms, or 10 pounds - some investigators initially said last week that they were relatively crude.
On Tuesday, the police searched five locations in West Yorkshire, in northern England, in the first known raids in Britain in connection with the bombings, Agence France-Presse reported.
"There are no arrests at this stage," a police spokesman said. "The searches are in connection with the terrorist attacks in London on July 7. The operation continues."
West Yorkshire includes former industrial hubs like Leeds and Bradford with large Muslim populations of South Asian origin.
On Monday, a senior European-based counterterrorism official with access to intelligence reports said the new information on the explosives material indicated that the bombs were "technically advanced."
The official added: "There seems to be a mastery of the method of doing explosions. This was not rudimentary. It required great organization and was well put together."
Counterterrorism and law enforcement officials interviewed for this article said they would only speak on the condition of anonymity because of the nature of the investigation. They said it was still unclear whether the attacks were carried out by local terrorists, a group from outside Britain or a combination of the two.
According to news reports Tuesday, the 13 people killed in the bombed-out bus in Tavistock Square may have included whoever was carrying the device.
"There are two bodies which have to be examined in great detail because they appear to have been holding the bomb or sitting on top of it," a senior police source said.
The quality of the explosives has led many investigators to theorize that the bombs were assembled by at least one technically savvy bomb maker, who might have come to Britain to build the devices for use by a local "sleeper cell," officials said.
"People assume you can look up a bomb-making design on the Internet and put one together without any training," said one senior counterterrorism official based in Europe. "But it's not that simple or easy."
Investigators say that determining the physical origin of the explosives is crucial to helping them determine the origin of the bombs the terrorists used last Thursday on three subway trains and the bus.
It was Britain's worst terrorist attack, killing at least 52 people. So far, only two victims have been formally identified.
British intelligence officials have asked their counterparts elsewhere in Europe to scour military stockpiles and commercial sites for missing explosives, three senior European-based intelligence officials said.
Senior officials are concerned that the terror cell that carried out the attacks might have a stockpile of more explosive material and could strike again, in Britain or in another European country.
"I really pity my British colleagues," a senior European intelligence official said. "It's a very difficult situation. Every hour that passes diminishes the probability to catch those people and increases the chances that this cell might try to strike again."
Britain's terrorism alert was raised immediately after the attacks to "severe specific," the second-highest level over all, and the highest that it has been since the Sept. 11, 2001, attacks in the United States. It has remained at that level since then, reflecting the continuing anxiety of the police and intelligence officials that another attack may occur in London.
In the attack on commuter trains in Madrid in March 2004, the industrial dynamite used for the bombs had been stolen from a quarry in northern Spain.
A month after the attack, investigators found the terrorist cell that was responsible. But the men blew themselves up in an apartment before the police moved in. Spanish officials said the members of that cell had obtained 230 kilograms, or 506 pounds, of Goma 2 Eco dynamite and had intended to build more bombs for additional attacks.
A senior Spanish official said Monday that roughly 130 kilograms of the explosive were used in the Madrid attacks, with about 30 kilograms of that in unexploded bombs.
The remainder is believed to have exploded when the terrorists blew themselves up.
A follow-up investigation last year determined that the police in Spain were informed in early 2003 that someone in northern Spain had been trying to sell a large quantity of explosives, but that the police had not done anything with the tip.
On Saturday, Andy Hayman, who is in charge of Scotland Yard's antiterrorism unit, announced that the four bombs set off in London each contained less than 4.5 kilograms of explosive material.
Hayman said investigators had determined by the shape of the twisted metal that the bombs had most likely been placed on the floor of the trains, near doorways. He said it was unclear whether the bomb on the bus was on the floor or on a seat.
British investigators believe the London bombs were equipped with timers, but they have not determined if the bombs were set off by synchronized alarms on cellphones or some other timing device, officials said.
By Don Van Natta Jr. and Elaine Sciolino The New York Times