The three bombs on London underground trains exploded almost simultaneously, police have said.
Scotland Yard said the attacks took place within 50 seconds of each other despite previously saying they had taken place over a longer time period.
British Transport Police (BTP) have also warned the recovery of victims could take “days more”.
Meanwhile, the government has announced a two minute silence in memory of those killed for 1200 BST on Thursday.
Indrukwekkende foto’s van de ontplofte bus (BBC.co.uk:
A BBC News website reader witnessed the immediate aftermath of the Tavistock Square bus bombing which killed 13 people. This picture was taken from Endsleigh Place, approaching the square.
Some witnesses ran from the scene, others rushed to help the injured.
Police say the bomb exploded on the top deck of the bus but this picture shows that many of those at the front were unscathed.
