That's true. However various things are making me more sure that an aircraft wasn't involved.
*incident was 2 hours ago, normally if a plane goes down there is confirmation from air traffic control by this time.
*Air traffic is still going in and out of Luton - if this was a terrorist attack with a plane, would have grounded all the others.
*Sky News have interviewed various witnesses who were quite close to the actual explosion - none of them have mentioned any possibility of a plane. The rumours have come from people miles away, some of whom only report 'a noise that sounded like a plane going overhead' - could well be that this was the beginning of the explosion.
CNN has it now.