A slightly different picture is emerging around the timing of these bomb incidents.

We are making an urgent appeal for any photos or videos -- including from mobile phones -- taken Thursday morning close to where the bombs exploded.

All three bombs on the London Underground system actually exploded within seconds of each other, at 8:50 in the morning.

But whether it is military explosive, whether it is commercial explosive, whether it is plastic explosive, we do not want to say at this stage.

There is a possibility that the person with the bomb died on the bus, there is a possibility they left the bomb, ... We're not ruling anything out, we're not ruling anything in.