The starting grid

The bikes lined up on the grid, the crowd on its feet and the anthem starting. The brothers sang and, at the track, it sounded perfect to them. The earpiece, meanwhile, was already somewhere else. Four people and an earpiece: 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

• Adelino, the rider of bike 33: He heard the anthem with his helmet on. He says it sounded fine, but everything sounds like an engine to him.
• Berta, the trackside photographer: She photographed the anthem up close. You can't hear anything in a photo, so they're all perfect.
• Custódia, the guest in the VIP area: She won the invitation in a raffle. She sang the anthem louder than everyone, and faster.
• Damásio, the circuit announcer: He introduced the anthem in his announcer voice. Then his microphone went quiet too, out of solidarity.