CTA video shows Sergeant Thomas O'Grady boarding the bus. It's not clear what happens afterward.
Mendoza says O'Grady attacked him and he filed a lawsuit against Chicago police. But prosecutors accuse Mendoza of lying. He was arrested and charged with filing a false police report.
Mendoza's union claims the charges are trumped up and that police have been trying to intimidate him by calling him and driving past his home.
"Psychologically he's a wreck... he was doing his job when he was attacked. It was unprovoked and it was brutal," said Marvin Jacobs, Amalgamated Transit Union Exec. Bd. Member.
Police Superintendent Jody Weis supports the charges against Mendoza. He said it sends "a powerful message to anyone attempting to cheat the criminal justice system."
