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Terrorists born or based in the UK were "almost certainly" behind the London bombs, a former Met police chief says.
Suggesting foreign attackers were to blame was "wishful thinking", ex Police Commissioner Lord Stevens told the News of the World.
The search for bodies trapped in train carriages below King's Cross is continuing, with workers facing extremely difficult conditions.
There have been 49 confirmed deaths, with a further 25 people missing.
At least 700 people were injured after three bombs exploded on trains on the London underground during morning rush hour, followed by a further bomb blast on a packed bus.
Training camps
Lord Stevens said it was thought 3,000 British born or based individuals have been trained in al Qaeda camps.
He said that authorities believed that either British individuals who had been through al Qaeda training camps, or others they had trained were most likely to have been the attackers.
He said suggesting people of other nationalities were responsible for the atrocities was "a damaging illusion".
Meanwhile, it is understood the UK's threat assessment has been returned to its highest level, having been lowered soon after the general election.
BBC security correspondent Gordon Corera said: "The raising of the level reflects real concern of another attack in the near future."
Overnight, up to 20,000 people were evacuated from Birmingham city centre during a security alert. The area has now been fully re-opened.
Religious leaders
Services around the country are expected to say prayers for the victims and for relatives and friends involved in searches for loved ones.
A service is to be held at a church near King's Cross station, where more than 20 people lost their lives.
There is still no certainty about the number of people whose bodies remain trapped in wrecked train carriages below King's Cross.
The recovery operation would be a "slow, methodical, meticulous process" in very difficult circumstances, said Deputy Chief Constable Andy Trotter.
There is expected to be a joint response from religious leaders around the UK.
The huge police investigation and forensic examination continues.
Police have said the explosives made were not home-made, but it is still unclear whether it or those who used it came from the UK or abroad, said Gordon Corera.
The police now say the Tube explosions took place within 50 seconds of each other, at 8.50am.
It had earlier been thought there was a wider time between explosions.
'Timing devices'
The synchronisation could suggest bombs used in the attack were triggered by timing devices.
A claim for the attacks has been made in the name of al Qaeda by a group calling itself the Abu Hafs al-Masri brigade.
But caution has been urged over the credibility of the claim.
Police are also involved in one of the UK's biggest searches of CCTV footage to see if there are any clues as to the identity of the bombers.
Families search for the missing
A 24-hour reception centre has been opened at the Queen Mother Sports Centre in Victoria, to help the families of people not seen since the explosions.
Those looking for missing people have been contacting hospitals, as well as taking photos and posters to the four blast sites.
An emergency call centre in London has taken more than 120,000 calls from the public.
Where the blasts happened
Scotland Yard confirmed seven people died in the Liverpool Street explosion, another seven at Edgware Road, a further 13 in the Tavistock Square bus blast and at least 21 at the King's Cross blast. A 49th person died in hospital later.
Of the 700 people hurt, about 69 are being treated in hospital and 15 remain in a critical condition.
A two-minute silence is to be held in memory of those killed at 1200 BST on Thursday.
hxxp://news.bbc.co.uk/1/hi/uk/4668675.stm
