A recent study of surveillance cameras in San Francisco found that cameras in public places led to a significant decrease in pickpocketing, purse snatchings, and theft from automobiles and buildings. However, the cameras appear to have little noticeable effect in limiting violent crime, drug offenses, prostitution, vandalism or crimes in non-public areas. Police investigators complained that the poor image quality limited the use of footage from the cameras primarily to establishing a sequence of events and placing a witness at a scene rather than as a ”silent witness” that can stand in for human witnesses.