"...one of the early signs that indicated what could be possible in the ensuing weeks"

I think that just about covers everything. I've just tried to imagine something that wouldn't be "possible in the ensuing weeks", and it's not that easy.

It occurs to me that it's pretty likely that any terrorist is going to be carrying a mobile phone, so we should definitely be keeping an eye out for that. On the other hand, these terrorists are crafty enough to know that we'd think that, so anyone when we come across someone who isn't carrying a mobile phone we should be very suspicious...