The WSJ is reporting that one of the Paris attackers had a ticket to the France vs. Germany match and tried to enter the Stade de France. The attacker detonated his vest, after it was discovered when he was frisked entering the stadium.
"While attempting to back away from security, Zouheir said, the attacker detonated the vest, which was loaded with explosives and bolts, according to Paris prosecutor François Molins. Zouheir, who was stationed by the players’ tunnel, said he was briefed on the sequence by the security frisking team at the gate. A police officer confirmed the sequence, adding that police suspect the attacker aimed to detonate his vest inside the stadium in order to provoke a deadly stampede."
A second attacker detonated his bomb outside the stadium. A third did so in a nearby McDonalds. Explosions were heard from inside the stadium during the match. An attack within the stadium and the subsequent panic could have made an already tragic evening far worse.