[ANDY72632]

The terrorists knew that they would be searched and may have installed or recruited a terrorist employee. I hope that the U.K. authorities thoroughly searched the stadium for suicide belts. They might be delivered in a supply order for the concession stands.

[Steve_Seattle]

Yes, you'd have to assume the terrorists knew of security procedures, so either they had confederates employed by the stadium who would let them in, or they figured they could conceal the bombs somehow, or they'd blow themselves up in the queue at the stadium entrance just before they'd be detected.