The broken alarm

The depot alarm was broken, everybody knew it and nobody was in a hurry to fix it. One night without power and the rest would be easy. Four people and one grenade: one person per row, one per column. Whoever is left alone with it in the same room is the one you're looking for.

Suspects

• Ademar, the alarm technician: He says the alarm had been broken for months. He keeps the repair request in a drawer.
• Bibiana, the radio operator: She hears everything on the radio. That night the radio was silent.
• Cândida, the acting commander: She was put in charge for a week. It was the wrong week.
• Dário, the electrician: He switches the generator on and off when needed. That night he switched it off once too often.