This is ****ing scary! (Well duh!) but this was exactly the plot in one of the episodes of the TV series
Sleeper Cell: the terrorists were taking an ambulance and bringing it into a football stadium to explode it, only it had nerve gas cannisters in it.
I've always feared that these scumbags and their ilk would attack a sports event. Think about it: thousands of people packed in together, no real way to escape in a hurry, everyone relaxed and off their guard, security generally lax (people smuggle in flares all the time) and if it's a national game, you're likely to have government ministers, media moguls, maybe even the head of state there. Not to mention that you probably have on-site TV coverage in place. So many juicy targets and all you have to do is get one guy in with a vest, or a grenade, or even a gun or knife, and the carnage that could ensue.
It's bloody terrifying.