Intelligence experts are continuing to scrutinise a video message by London bomber Mohammed Sidique Kahn to find out when and where it was made.
Security officials are also looking for clues as to whether a wider terror network was behind the 7 July attacks.
The video, shown on Arab TV network al-Jazeera, showed Khan criticising British foreign policy and saying he was a soldier fighting a war.
Relatives of some of the victims have expressed their outrage at the message.
Khan, 30, of Dewsbury, West Yorkshire, killed six people and injured 120 in the Edgware Road Circle Line explosion.
In the videotape he said the public was responsible for the atrocities perpetuated against his "people".
He also said he was inspired by al-Qaeda leader Osama bin Laden and the reputed leader of the terror group in Iraq, Abu-Musab al Zarqawi.
It was recorded on the same video as a message from al-Qaeda's Ayman al-Zawahri, also shown on al-Jazeera.
Friends of Khan told BBC News it showed him looking significantly different than he did immediately before 7 July.
They believed the message had been recorded some weeks or months before the bombings.
In the video Khan blamed the public for democratically electing governments who carried out "atrocities" around the world.
"Until you stop the bombing, gassing, imprisonment and torture of my people we will not stop this fight," he said.
"We are at war and I am a soldier. Now you too will taste the reality of this situation."
UK Foreign Secretary Jack Straw said there was "no excuse for terrorism", adding that a security assessment of the tape was under way.
But the leader of one of Britain's biggest mosques told BBC Radio 4's Today programme the tape did not prove the bombings had been carried out by Muslims.
Dr Mohammed Naseem, chairman of Birmingham Central mosque, said it could have been doctored.
He called for an independent inquiry to establish whether the bombers were Muslims, saying the situation was "not clear".
He said: "We are in the 21st Century. The cows can be made to look as dancing, the horses can speak like humans, so these things can be doctored or can be produced."
BBC security correspondent Gordon Corera said counter-terrorism sources were not treating the messages as conclusive proof that the al-Qaeda leadership directly ordered - rather than simply inspired - the attacks.
John Taylor, who lost his daughter Carrie in the attacks in which 52 people were killed, said the videotape showed the four bombers were "evil".
Gous Ali, whose girlfriend Neetu Jain died, said Khan's message was "lies".
The Hindu girlfriend of Mr Ali - himself a Muslim who has studied Islam - was killed in the explosion on the number 30 bus in Tavistock Square.
He said: "It's all brainwashing by some wacko scholar who believes his own version of the Koran and has made it his own battle. There is no holy war.
"They have so much coverage it's damaging, yet the voices of the innocent victims are not being heard."