One of those that caused it ...
The Times has the story of a passenger who cheated death, and perhaps also of the one that might have tried to cause it:
Terence Mutasa, a staff nurse at University College hospital, treated two passengers, young women in their twenties, for minor injuries and shock. “They were saying some guy came and sat down on the bottom deck and that he exploded,” he said. “They said the guy sat down and the explosion happened. They thought it was a suicide bomber.”A report from the Guardian corroborates the claims, which are still perhaps only speculation. Other reports have wondered whether it was a suicide bombing, or a bomb that exploded early, which fits with Guardian reports that the devices were not exactly reliable.
But a source told The Guardian that three controlled explosions had been carried out on "suspect devices". Furthermore Vincent Cannistraro, the former head of the CIA's counter-terrorism centre, told The Guardian that "two unexploded bombs" were recovered as well as "mechanical timing devices".
An earlier BBC-TV broadcast suggested one of those was apparently at Stockwell station; if so it suggests South London was also on the list of targets.