But the instability of a tunnel between King's Cross and Russell Square Stations meant they were having difficulty at one of the bomb scenes.
'There is great difficulty in deciding or determining how many fatalities because two of the scenes are very difficult in terms of recovery.
'One is the bus because of the nature of the explosion and more acutely the Tube train at Russell Square continues to hold a number of bodies. We don't know how many are there.'
Sir Ian said that should not be taken as a suggestion that the final number could be in three figures.
Assistant Commissioner Andy Hayman said: 'It's yet to be the case for us to get near the carriage. There is a risk of the tunnel being unsafe. I ask everyone's patience as we progress this matter.
'It would be wholly unwise and could inhibit a successful prosecution if we rushed this stage.'
The ones on the Tube were likely to have been placed on the floor of the carriages, he said.
Police have established that the Liverpool Street explosion happened 100 yards into a tunnel and the device was in the third carriage of the train.
In the explosion between King's Cross and Russell Square the bomb was in the first carriage in the standing area by the first set of double doors.
For the Edgware Road blast the device was placed in the second carriage of the train in the standing area by the first set of double doors.