Today, there are barely any signs of the protests in central Xi’an. Witnesses said it took a few days for city workers to clean up a pool of Mr. Li’s blood, but the street is back to normal. People looking closely at cars, though, can see that there are now small Chinese flags affixed to many of them, in particular those of Japanese make.

Many liberal Chinese and foreigners have been asking whether the central government stoked the protests, or at least allowed them to go on unabated for too long. At the protests in Xi’an, which were among the most violent, the police were out in force, but some officers did not try to stop rioters from overturning police cars of Japanese make, several witnesses said. The attack on Mr. Li took place within 500 feet of a police station.

Mr. Li was driving home with his wife on Sept. 15 in a white Toyota Corolla after they had spent the morning looking at construction materials for their son’s new apartment, Ms. Wang said. Not far from the western gate of the ancient city wall, they encountered a crowd of protesters waving Chinese flags, chanting slogans and smashing cars.

Protesters quickly encircled the Corolla and used batons, bricks and bicycle locks to smash the car with the couple inside.

The two stepped out of the car to plead with the protesters to stop. Details of what happened next are murky, but the Internet video shows a heavyset man leaping at Mr. Li and delivering four blows to the back of his head with a bicycle lock. In the video, the sharp sound of metal whacking against skull can be heard above the din of the crowd. Ms. Wang screamed for help as she sat on the ground trying to stop the blood flowing out of her husband’s wound.