PARIS — Gendarmes in Brittany on Wednesday arrested a fan involved in a massive pileup at the Tour de France during the opening stage at cycling’s biggest event, local media reported.

The fan brandished a large cardboard sign while leaning into the path of oncoming riders. She appeared to be looking in the other direction, apparently at a camera, and not at the approaching peloton.

The woman, not publicly identified, was arrested by gendarmes in the Finistere region who tracked her down based on “solid” accounts from people questioned this week, France Bleu Finistere said, citing a source close to the probe. Investigators had spoken to dozens of people since the incident on Saturday, the local radio station said.

Tour organizers had announced after the crash on the stage from Brest to Landerneau that they would start legal proceedings against the fan, who disappeared from the crash scene. She had leaned into the path of veteran rider Tony Martin, who fell off his bike and took dozens of others down in his slipstream. German rider Jasha Sutterlin was forced to abandon the race.

The Gendarmerie du Landerneau, east of Brest, had put out a call for witnesses shortly after the pileup. It refused to comment on the reported arrest.

Fans gathering on the sides of roads and in villages as riders pass by is part of the tradition and charm of the Tour. But the woman in question leaned into the path of cyclists with her sign that read “Allez Opi-Omi,” a mix of French and German-language terms of endearment for grandparents — “Go Grandpa-Grandma.”

Oblivious Fan Causes Enormous Crash, Ruins Tour de France from facepalm