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, some investigators initially said last week that they were relatively crude.
On Monday, a senior European-based counterterrorism official with access to intelligence reports said the new information on the 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.
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 counter terrorism official based in Europe. "But it's not that simple or easy."
Investigators say determining the physical origin of the explosives is crucial to helping them determine the origin of the bombs that tore apart three trains in the London Underground and the No. 30 bus in central London during the morning rush hour last Thursday. It was the worst terrorist attack in Britain since World War II.
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 counterterrorism officials are concerned that the cell that exploded the bombs 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 overall, 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 here 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 the cell had obtained 230 kilograms (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 (286 pounds) were used in the Madrid attacks, with about 30 in unexploded bombs. The remainder is believed to have exploded when the terrorists blew themselves up. The terrorists had obtained the dynamite from a man named José Emilio Suárez Trashorras, who was arrested shortly after the bombings.
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 10 pounds, or 4.5 kilograms, of explosive material. Mr. Hayman said that 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.
Initially, investigators contended that the bombs, outfitted with timers, had gone off at different times; they thought 26 minutes separated the first bomb to explode in the Underground from the third bomb. On Friday, some investigators said that they believed the bombs were crude devices, possibly even homemade.
But on Saturday, Scotland Yard said that a reassessment showed that the three bombs in the Underground blew up within 50 seconds, about 8:50 a.m. The synchronized explosions suggested that the plan might have been more sophisticated than investigators initially believed. Police officials also announced Saturday that the bombs were "high explosives," but they declined to elaborate.
Now, senior British and other European investigators say they are convinced that the cell responsible for the bombings had executed a well-thought-out plan. One official said the cell's attack plan was "highly sophisticated" and "meticulously planned."
Investigators said they had reached their conclusion in part because the devices were powerful enough to blow apart several coaches of the trains and rip the roof off a red double-decker bus in central London.
"The only concrete evidence is that these are not homemade," a European-based senior official said. "We don't know if they are civil industrial or military industrial explosives." Britain has one of Europe's best security systems for warehouses containing explosive materials, specialists say.
British investigators are being helped with the slow forensics work by a teams from the United States, Spain and France. But Britain has a lot of experience doing such work.
In the 1990's, the Irish Republican Army used Semtex B, a Czech-made substance that is often nearly impossible to detect. British antiterrorist police discovered that the bombing of London's Canary Wharf district used Semtex B, and it was enough for then to conclude the I.R.A. was behind the bombing.
A Spanish official, who spoke on condition of anonymity, denied a Spanish press report that suggested that 80 pounds of bomb material was still missing from the dynamite used in the Madrid train bombings.