LONDON – The probe into the London suicide attacks shifted overseas Friday as officials in Cairo announced the arrest of a wanted biochemist and authorities searched for a Pakistani man suspected of helping in the plot and leaving the country the day before the bombings.

A separate search continued for a man whom a security videotape shows talking with the bombers in a train station the morning of the attacks, intelligence sources said.

A week into the highly secretive probe, investigators appeared to be theorizing a conspiracy of at least seven people – the four bombers plus these three possible accomplices. In Pakistan, they said, there might be more.

Counterterrorism officials in that country confirmed Friday that two of the bombers visited Pakistan separately in the past year and apparently met with members of radical Islamic networks. Friends have said a third bomber also traveled to the country. These three of the four bombers were of Pakistani heritage.

The hunt for the man who spoke with the bombers began after British intelligence reviewed a surveillance video from Luton rail station north of London and saw the four attackers huddled in conversation with another man on the morning of the bombings, British and U.S. intelligence sources said Friday.

British intelligence has interviewed a witness who was at the station at the time, according to the sources. The witness told authorities that when the conversation among the men ended, the four now identified as the bombers walked away together, while the other, still-unidentified man left alone in the opposite direction and boarded a train.

In Cairo, Egyptian officials said they arrested Magdy Nashar, a biochemist at Leeds University who is wanted by British authorities for questioning in the attacks. Residents in Leeds said Nashar had helped in the rental of a Leeds townhouse near the university where police found explosives after a raid Tuesday. Officials said they suspect the house was used to assemble the bombs used in London, although investigators said they are unsure whether Nashar was aware of the plot.

Associated Press

A woman prays in front of a flower tribute to those killed in the July 7 attacks during a ceremony Friday at Russell Square Gardens in London.