Thirty key al-Qa'ida-linked terror suspects are identified by police

The mixture of foreign and British-born suspects are being investigated as part of a massive inquiry to track down those responsible for last Thursday's attacks. The cell responsible is believed to number from four to 12 people, anti-terrorist sources have revealed.

The sources also disclosed that an initial group of about 30 key suspects has been identified from the hundreds of pieces of intelligence collected by MI5, MI6, Scotland Yard's anti terrorist branch, and the bugging centre at GCHQ (the Government Communication Headquarters) in Cheltenham. The material includes phone taps and reports from foreign agents and police forces.

Among the suspects being investigated is the terrorist believed to have organised last year's Madrid train attacks in which 191 people were killed. Spanish intelligence officers have flown to London to help the police. They were among about 100 senior police officers from forces across the world that gathered at Scotland Yard in central London over the weekend to share information.

British intelligence officers are concentrating on trying to identify an individual bomber, or one of their supporters, which they hope will lead to the rest of the terror unit. "It is from investigating individuals that you find out who they are associated with and how they operate," said a counter-terrorist source. "The unit involved could be anything from three or four to about a dozen," the source added.

Among the 30-odd key suspects being investigated by British intelligence officers is Mustafa Setmariam Nasar, a 47-year-old Syrian, who is believed to be in hiding in Iraq or on the Afghanistan-Pakistan border.

Nasar is considered to be a likely suspect because he has lived in London and has contacts there going back 10 years. He is known to have organised terrorist training camps in Afghanistan and is believed to be the mastermind behind the Madrid train bombings.

MI5 and anti-terrorist police have previously identified British-born Muslim extremists who had the training and skills to make high-explosive bombs similar to the ones detonated on the Tube and on a bus in London. At least one such individual has been arrested by the authorities.

It is too early to know whether the terrorists responsible for Thursday's atrocity are British-born Muslims or foreign al-Qa'ida fighters, according to security sources. "The modus operandi that we know so far doesn't give us an awful lot to go on. They don't tell us whether they are a homegrown radicalised unit or overseas grouping, or whether they are mixed. It is too early to make that judgement," a security source said.

Most terrorist experts believe from the initial details of the attacks that an experienced bomber with skills, probably gained from an al-Qa'ida training camp, was involved. But Lord Stevens, the former commissioner of the Metropolitan Police - then Sir John - predicted yesterday that the London bombers were "almost certainly" British. Lord Stevens, who served as commissioner for five years before retiring this year, said that the bombers were "totally aware of British life and values" and although international terrorists may have provided the expertise, it was "wishful thinking" to suspect the perpetrators came from abroad.

He said: "I'm afraid there's a sufficient number of people in this country willing to be Islamic terrorists that they don't have to be drafted in from abroad."

He continued: "[The bombers] will be apparently ordinary British citizens, young men conservatively and cleanly dressed and probably with some higher education. Highly computer literate, they will have used the internet to research explosives, chemicals and electronics.

"They are also willing to kill without mercy - and to take a long time in their planning."

He added: "We believe that up to 3,000 British-born or British-based people have passed through Osama bin Laden's training camps over the years. Plainly, not all went on to become active Islamic terrorists in the UK."

Meanwhile, the police are continuing their forensic investigation, which is concentrating on trying to recover fragments of the bomb and traces of the explosives from the four blast sites.

Detectives revealed that the three bombs on Tube trains at Aldgate, Edgware Road and King's Cross had exploded almost simultaneously at 8.50am. Technical data from London Underground showed there was a gap of about 50 seconds between the first and third explosions.

The later bomb on a No 30 bus at Tavistock Square in central London, which killed 13 people, went off nearly an hour later at 9.47am. It is unclear whether the bomber was still on the bus. It is also not yet known if it was a suicide bomber, if the device went off by accident, or whether there was a timer set an hour later than the Tube devices.

The police issued an urgent appeal yesterday to the public for any photographs, video footage or mobile phone images taken in the aftermath of the bombings. They should be e-mailed to Scotland Yard at: imagesatmet.police.uk.

* Three men were arrested yesterday morning under anti-terror laws at Heathrow airport. The three British nationals were detained after they were turned back from the United States, but police sources said they were not being linked to Thursday's bombings. The three men are expected to be released overnight.