PARIS—At least one of the attackers outside France’s national soccer stadium had a ticket to the game and attempted to enter the 80,000-person venue, according to a Stade de France security guard who was on duty and French police.

The guard—who asked to be identified only by his first name, Zouheir—said the attacker was discovered wearing an explosives vest when he was frisked at the entrance to the stadium about 15 minutes into the game. France was playing an exhibition against Germany inside.

While...