At first few people seemed to notice the encounter. The young Parisian woman in red and the man in the black shirt exchanged words as they passed each other in the street. They went their separate ways.

But then, she said, he threw an ashtray in her direction. She shouted at him. He swiveled around and followed her. Then, in front of more than a dozen people sitting at a cafe, he hit her, she said.

The woman, Marie Laguerre, a 22-year-old engineering student, posted the cafe’s surveillance video on social media the next day. There was no audio, but it showed her receiving a blow to the head as other patrons at the cafe watched in shock. In the video, although the man in the black shirt was only partly seen on camera, it appeared as though he had hit her.