The Egyptian biochemist lived near a "bomb factory" which police found. But his activities were also under scrutiny by the FBI when he was at university in Raleigh, North Carolina.
Mr El-Nashar was arrested at his family home in the middle-class Maadi district of Cairo after a request by Britain through Interpol. He had left his address in the Leeds area, home of three of the bombers, six days before the blasts. In Cairo, the Egyptian interior ministry said Mr El-Nashar had denied involvement and has insisted he intended to return to the UK after a 45-day holiday.
Other international connections are accumulating. In Islamabad, Pakistani authorities are investigating alleged links between one bomber, Shahzad Tanweer, and an Islamist group, Jash-e-Mohammed, which has ties to al-Qa'ida and has mounted terrorist attacks in Kashmir. It is banned in Pakistan.
On a visit to Pakistan two years ago, Tanweer is said to have met Omar Nazir who was arrested last year for bombing a church in Islamabad that killed, among others, two Americans.
And names on a computer seized last year from Mohammed Naeem Noor Khan, a Pakistani al-Qa'ida suspect, allegedly showed foreknowledge of the London attacks.
Tanweer is said to have studied at Muridke, a madrassa, or religious school, in Lahore run by a group called Jamaat-ud-Dawa. British authorities are following up reports that Mr El-Nashar also attended a madrassa in Pakistan.
The Egyptian chemist is reported to be an associate of Jamaican-born Lindsay Jermalne, named as the fourth bomber. Friends of Mr El-Nashar say they met him with 19-year-old Jermalne at the Grand Mosque in Burley opposite the house found to contain the bomb-making equipment.
Abed Shad, an architect who said he was a friend of Mr El-Nashar, said he had met the Egyptian with Jermalne at a mosque in Leeds.
Mr El-Nashar is said to have arranged for a "friend" to stay at the flat at 18 Alexandra Grove, Burley, through a doctor, Shakir Al Ani whose distant cousin, Samir Al Ani, had left the flat and moved to Iraq two months ago.