It was the end of another day of violent demonstrations in Hong Kong. The tear gas had dispersed and the crowds that had filled the streets were gone. The few demonstrators who remained were scattered around a popular shopping mall and getting ready to leave. Then, a group of men dressed in black rushed in, tackling people and beating them with batons.Protesters have accused the Hong Kong police of using excessive force throughout the demonstrations that have gripped the city for the past four months. But on the night of Aug. 11, a major shift occurred. For the first time, officers disguised as demonstrators were seen beating protesters and conducting arrests.Videos of the night went viral. They showed undercover officers hitting protesters with batons and pinning them to the ground, leaving some bleeding profusely. We analyzed footage of the night and spoke to more than a dozen witnesses and protesters who were detained. Lawyers and human rights advocates who watched the images say the police used excessive force to conduct arbitrary arrests.The Hong Kong police said they had conducted a “decoy operation” targeting a “core group of violent rioters.” But three of the men arrested said they did not know one another, and protests in the area had ended hours before the clash.One man says he suffered a brain hemorrhage; others had serious bone fractures. Doctors described one injury, a broken arm, as caused by assault. The episode became one more example of police tactics that have infuriated citizens, driving calls for an independent investigation into police misconduct.When asked about the footage of one of the bloody arrests made that night, Steve Li, senior superintendent of the Organized Crime and Triad Bureau, said that “officers used appropriate force to subdue the man and conduct an arrest.”Demonstrators said that undercover officers didn’t identify themselves as police, adding to fears that the men had been part of gangs that had attacked demonstrators in recent weeks. According to the Hong Kong police’s guidelines, officers are required to identify themselves before exercising their duties.