Gradually a picture is starting to build up. There was no lone suicide bomber. There was no one terrorist acting on his own. The probability is that the worst terrorist attack in Britain was carried out by at least a four-person cell using synchronised explosive devices attached to timers.
And Britain might not have been the only target. There is evidence that atrocities were also being planned in Italy and possibly in Denmark.
Such knowledge will not cheer the 100 senior police officers, drawn from forces across the world, who this weekend descended on the headquarters of the Metropolitan Police in London in a bid to share intelligence on the attacks.
But it will help the intelligence agencies start to obtain a clearer idea of what they are up against - and who they are looking for.
The unprecedented meeting of the world's senior police officers will be chaired by the Metropolitan Police's Assistant Commissioner, Andy Hayman, the man who is heading the investigation into the London bombings.
'You can never over-estimate the value of a briefing from overseas partners,' Hayman said yesterday. 'We wanted to be joined up because this type of terrorism affects the world.'
Officers from 32 countries including France, Italy, Australia, Israel, America, the Czech Republic, Japan, the Netherlands, Turkey, the United States and Switzerland are attending the summit. They also include a delegation from Spain, whose officers are hoping to share knowledge gleaned after the Madrid bombings.
One security source said: 'We learnt after Madrid that it is important to get a scrum down early on and swap information.'
The emergency meeting comes after Italian police yesterday arrested 142 people as part of an anti-terror operation in Milan, which was launched almost immediately after the London attack. One and a half kilograms of explosive were recovered in raids involving more than 2,000 Carabinieri and directly aimed at boosting security at underground stations in the city.
Later it emerged that an unknown al-Qaeda group, which last week claimed responsibility for the London bombings on a website, had threatened Italy and Denmark with similar attacks, although its credibility has been questioned.
Meanwhile, in Dubai a group claiming links to al- Qaeda also revealed in an internet statement that it was behind the London attacks. 'A group of mujahideen from the Abu Hafs al-Masri Brigades delivered one blow after the other in the infidels' capital, the capital of the English,' said the statement signed by the group. It is the third such claim by different groups.
Yesterday police revealed that, contrary to earlier reports, the bombs on the underground had not occurred over a 30-minute period as had been previously thought. There had been speculation that this meant a solo bomber could have placed the devices on the tube trains.
But after analysing technical data provided by the London Underground, the police confirmed the three devices detonated within moments of each other.
'We can clarify the position that the three bombs exploded almost simultaneously,' said Scotland Yard Deputy Assistant Commissioner Brian Paddick.
'Maybe that lends more towards timing devices than people actually with the bombs manually detonating. But we are not ruling out either possibility,' Paddick said. He declined to comment on whether a timer had been found at the scene of one explosion.
Privately, intelligence experts say they now believe the London Underground data suggests at least four terrorists were at work last week. The fact that the type of explosives used was not hand-made, but small 10lb commercial high explosives, probably linked to a timing device, suggests a level of sophistication and financing.
MI5 is combing through its files and is believed to be focusing on almost 100 suspects who could have carried out the operation.
Meanwhile experts at Qinetiq, the government's former research agency, are to begin creating computer simulations that will show the locations of where the bombs were planted and the directions of the blast.
Their work has been made all the more crucial by the revelation that the CCTV system on the Number 30 bus had not been working since June. The broken camera is bound to cause consternation over the condition of the 6,000 CCTV cameras installed on the underground. Around 500 officers are now helping to scrutinise the footage.
Yesterday forensic experts continued to pore over the sites of the four bomb blasts. The government's Forensic Science Services agency has removed thousands of samples to its laboratory in Birmingham and several Metropolitan Police facilities in the capital for processing.
But the forensics work is hampered by the grim conditions in the Piccadilly underground system, located 500 metres from King's Cross. The tunnel near Russell Square in London's Bloomsbury district remains unsafe in the immediate area around the blast. Engineers are concerned that the crucial steel lining that strengthens the tunnel, which has been bored through clay, may have been ruptured in the blast. They are currently considering a plan to drag several carriages down the track to obtain access to the wrecked first carriage in the train where the bomb went off. It is believed that more than 20 bodies are still trapped in the wreckage.
A specialist team of senior police officers, coroners and medical experts is now trying to ensure that the bodies are correctly identified.
The daily meetings of the newly formed Identification Commission will take place at an undisclosed military site in central London, where a temporary mortuary has been set up.
Led by Westminster coroner Dr Paul Knapman, Scotland Yard's senior identification manager, Detective Superintendent Jim Dickie, and Home Office pathologist, Rob Chapman, the commission will be supported by a team of hundreds of police staff and other experts, many of whom have experience of major terrorist attacks or natural disasters, including the devastating Asian tsunami on Boxing Day.
The commission will oversee a painstaking and complex identification process using fingerprints, dentistry and DNA to conclusively identify the victims. Experts said that it may take weeks to identify some of the bodies. Police are currently checking the details on their Holmes missing persons database.
Meanwhile a 24-hour, central London support centre has also been set up to offer practical and emotional support to those whose relatives have been killed or are missing.
The centre in the Queen Mother Sports Centre at 223 Vauxhall Bridge Road is manned by staff from the British Red Cross, Victim Support, Salvation Army, Cruse Bereavement and social services.