The family of 18-year-old London bomber Hasib Mir Hussain say they are "devastated" by what has happened.
In a statement they described the bus bomber as "a loving and normal young man who gave us no concern".
"We had no knowledge of his activities and had we done, we would have done everything in our power to stop him."
Earlier, an Egyptian chemistry expert sought by police over the attacks, Magdi Mahmoud al-Nashar, 33, was arrested in Cairo.
Hussain was reported missing by his family in the aftermath of the bombings.
We, the family of Hasib Mir Hussain, are devastated over the events of the past few days," they said in Friday's statement.
"We are having difficulty taking this in.
"Our thoughts are with all the bereaved families and we have to live ourselves with the loss of our son in these difficult circumstances.
"We had no knowledge of his activities and, had we done we would have done everything in our power to stop him.
"We urge anyone with information about these events, or leading up to them, to co-operate fully with the authorities.
"This is a difficult time and we ask you to let us grieve for our son in private."
Muslim leaders and scholars met at London's largest mosqueon Friday and condemned the attacks, saying the bombers had violated the Koran by killing innocent civilians, and could not be regarded as martyrs.
"There should be a clear distinction between the suicide bombing of those who are trying to defend themselves from occupiers, which is something different from those who kill civilians, which is a big crime," said the head of the World Islamic League's Sayed Mohammed Musawi.
A statement by imams and scholars said unemployment and racism "may be alienating some of our children and driving them toward the path of anger and desperation" in ways contradictory to Islam's teachings.
Earlier, police searched a house in Leeds linked to Mr al-Nashar, who was arrested in Cairo, but did not formally named him as a suspect in their investigation.
The 33-year-old had not been seen by colleagues at Leeds University since early July.
He denied having any role in the attacks and said he was on holiday, Egypt's Interior Ministry said.
Searches continue
Police are hunting the financiers, supporters and chemists who assembled the bombs which killed 51 people in London on Thursday.
Three of the four bombers are also confirmed dead, and the fourth bomber is also thought to have died.
Mr al-Nashar's alleged link to the plot is unclear, but it was believed he handed over keys to a house in Leeds used by the bombers, said BBC security correspondent Gordon Corera.
He has completed a PhD in chemistry at Leeds University.
Police searching homes in Leeds and Buckinghamshire, a community centre and Islamic bookshop near the home of one of the bombers, Shehzad Tanweer.
Explosives made from ingredients available from high-street chemists were found in one of the Leeds homes, sources have told the BBC.
They were the same kind of explosive Richard Reid had in his shoes when he tried to blow up a trans-Atlantic flight in 2001.
The BBC has also learned a suspected al-Qaeda member entered Britain via a Channel port two weeks before the bombs, but was not kept under surveillance.
The man apparently left Britain hours before the blasts.
Met Police commissioner Sir Ian Blair said there was nothing connecting the man to the plot, but added that investigators expected to find evidence linking al-Qaeda to the attacks.
He said the four bombers were only the "foot soldiers" of the operation and the mastermind was still being sought.
"What we've got to find is, who encouraged them, who trained them, and who's the chemist," he said.
In other developments Friday:
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