After a terror attack on the scale of the London bombings, where do the police begin in tracking down the perpetrators?
Forensic experts will examine the blast scenes in London
Forensic evidence, which will be a key element of the investigation, has already begun to reveal information about the bomb attacks that have claimed more than 50 lives in the capital.
The police say that they believe the bombs contained less than 10lbs of explosives - which could have been carried in a bag or rucksack.
The devices used high-explosives and were not home-made, which will also be taken as an indication of whether these were "home-grown" terror suspects or those with international links.
It has also been revealed the three bombs on Tube trains exploded at almost the same time.
Scotland Yard Deputy Assistant Commissioner Brian Paddick, struck a cautionary note about such details.
"Clearly, there are two possibilities here. Either you have people with the explosive devices who synchronised watches or whatever, and they have simultaneously detonated their devices at the same time.
"Or it could be these devices were triggered by timing devices that were co-ordinated to go off at the same time," he said.
Commissioner Sir Ian Blair has already said the attacks had "all the hallmarks of al Qaeda" - but he said there was nothing so far to suggest the bus bombing was a suicide attack.
At the site of the attack on the bus in Tavistock Square, a fingertip search is being carried out, say the police.
The roof of the bus, which was blown off in the blast, has been taken away for examination.
Part of the forensic operation will be the identification of victims, with the police saying this could include the use of DNA.
After the painstaking process of removing the victims' bodies from the bomb sites has been completed, the train carriages, still in place below ground, will be the subject of forensic investigation.
Police will also be searching through the huge amount of CCTV footage from the areas of central London surrounding the explosions - whether from shops, businesses, public buildings, Tube stations or traffic cameras.
Professor Hans Michels, an explosives expert at Imperial College London, said the scientists would be gathering clues about the source of the explosion from the way objects have been deformed.
"There will be an intense burning mark where the blast happened"
"The next thing to do would be to carefully pick up all the bits and pieces, take many, many photographs and store items in envelopes and plastic bags, sending samples off for analysis."
From the levels of deformity, scientists would try to work out how much energy was released in the explosion and what type of explosive was used.
Identifying the explosive needs only a tiny sample - one millionth of a gram of nitroglycerine, for example.
Wreckage from Lockerbie included parts of the bomb
Lawyers prosecuting Oklahoma bomber Timothy McVeigh linked the fact that ammonium nitrate had been used in the explosion with a receipt for one ton of the same substance that had McVeigh's fingerprint on it.
Gloria Laycock, director at the Jill Dando Institute of Crime Science, agreed forensics would be crucial.
"They will try and identify what kind of explosive it was and link the explosive to the other sites to see if it came from the same source.
"Traces are usually left behind, more than people think and it does amaze me what they do manage to reconstruct.
"Crime scenes need to be preserved to get the best evidence they can, but if the Tube system needs to be open, that could be difficult."
The Met's most experienced anti-terrorist officers are already involved in the investigation and Sir Ian said the force had been "overwhelmed by offers of help from Europe and the US".
Professor Laycock believed IRA bombing campaigns in the past meant the Met had the expertise to investigate the attacks itself.
However, help from overseas forces could be important once the evidence was gathered.
If the bombers were identified to be foreign nationals, then the Met would seek help and co-operation from Interpol and police in the countries concerned, she said.
If the perpetrators were suicide bombers, she said, then Israel might be able to provide some expertise because there were certain patterns of behaviour which suicide bombers followed.
After an explosion, the wreckage itself can also be important.
The search for evidence from the Pan Am jet, which exploded over Lockerbie, was exhaustive but from it investigators found scraps of clothes and tiny bits of the bomb and its casing.
Together this formed a significant part of evidence in the trial of the two suspects, one of whom was eventually convicted.
However, experienced terrorists can combat forensic methods.
They try to cover their tracks after handling explosive material by disposing of any shoes and clothes worn while putting the incendiary device together. They may also try to wash specks off their body and even cut their hair.