One of the 7 July London bombers rang his fellow suicide attackers before his device went off, the BBC has learned.
Hasib Hussain, 18, called his three accomplices on his mobile phone before killing 13 people on the No 30 bus.
But Mohammad Sidique Khan, 30, Germaine Lindsay, 19, and Shehzad Tanweer, 22, had already killed themselves and 39 passengers on three Tube trains.
No-one has been charged over the 7 July bombings. The main suspects in the 21 July failed attacks are all in custody.
Three bombs exploded on the London Underground at around 0850 BST on 7 July, but Hasib Hussain did not detonate his device until 0947 BST.
Scotland Yard has confirmed to BBC home affairs correspondent Daniel Sandford that, at some point in the intervening hour, Hussain tried to contact his accomplices.
He did not try to contact anyone else, which suggests there was no "mastermind" or that he did not want to implicate them, said Daniel Sandford.
The most likely explanation was that he had tried to board the Northern Line, but it was closed, so when things started to go wrong he tried to phone his co-conspirators, he added.
Terrorism expert Michael Clarke, of King's College, said the calls showed Hussain was in a panic and probably rang his accomplices to make sure their bombs had exploded.
The 7 July bomb attacks killed 52 bus and Tube passengers and injured a further 700 people.
On 21 July no-one was killed when bombs on three Tube trains and a bus failed to detonate.
So far 14 people have been charged over the 21 July attempted attacks, including three of the suspected bombers.
A fourth bomb suspect, Hussain Osman, 27, is awaiting extradition from Italy.