The July 7 extremists received bomb-making instruction from a mystery figure in Pakistan, an inquest has been told.
Members of the group, who perpetrated the worst single terrorist atrocity on British soil, carefully constructed their home-made devices using a cocktail of concentrated hydrogen peroxide and pepper.
Lacking the expertise themselves, they are believed to have been guided by an unidentified individual in Rawalpindi.
Mobile phone records analysed in the wake of the bombings revealed a series of calls made from phone boxes in Pakistan to ringleader Mohammed Sidique Khan.
One of these, on July 2, 2005, five days before the devastating explosions, lasted for six minutes, Detective Sergeant Mark Stuart of the Metropolitan Police told the inquest at London’s Royal Courts of Justice.
Hugo Keith QC, counsel to the inquests, asked him: “Did you asses that those calls therefore were probably connected to some guidance or some means of communicating information concerned with the manufacture of the bombs and then ultimately their detonation?”
“Yes, I think they had to be,” he replied.
Many of the calls, though made from different phone boxes, were made within minutes of each other, suggesting whoever phoned Khan was intent on concealing their identity, the inquest was told.