Joint VentureA person can be found guilty of a criminal offense even if he did not personally commit the crime, but instead, aided and abetted its commission as part of a 'joint venture' with other persons.
The following article (very thorough and informative) says the bus station tape may be obscured (you'll have to cut and paste, I can't link):
http://www.centredaily.com/2011/08/06/2870668/hundreds-protest-fullerton-mans.html
"Fullerton's use-of-force policy explicitly directs officers to intercede if they see fellow officers applying what they believe to be excessive force."