SAN FRANCISCO—A videotape found more than a year ago by a defense lawyer in a routine drug case has helped spark this city's biggest police-misconduct probe in years—one of numerous investigations into law-enforcement practices being conducted nationwide by the U.S. Department of Justice.

The tape, attorney Scott Sugarman says, showed his client dressed in a black jacket as police arrested him. But officers wrote in statements that the defendant was wearing a white jacket that contained drugs. The drug-possession case was...