If 80,000 people at a football game try to use their cell phones at the same times, how many open circuits does that take, and how many calls can one cell station allow through...
Not necessarily shutting down of computers... Sometimes computers and cell coverage crashes from overload...
When 9/11 happened, I knew someone who tried to call a friend in New York City (from Atlanta). He thought because he could not get through the person was dead.
It was simply a case of jammed circuits.
They might shut things down just so they really don't foul up communications for the people who really need it in such cases...
Or computers could crash.
Don't you think it is possible that 80,000 people might have tried to use their cell phones after the blast?
I would imagine overload did cause the lack of signals... at least to start with. Police have emergency access to phone personal that could then shut down the needed towers quickly (2-5 minutes tops) by simply pulling the tower(s) from the network. After the crisis or whatever passes the tower(s) are added back into the network and will start working immediately. Probably was a combination of these two things.