Detectives say 29-year-old Mylyn Johnson, a registered sex offender from Lancaster, would use his cellphone to do the recording, and they've found more than 3,200 videos so far.
"It's pretty egregious behavior," said Los Angeles County Sheriff's Sgt. Brian Hudson. "It's violated some privacy of a lot of women, over 100 at this point."
Ten victims have been identified so far, but detectives say there are 100 other women they want to track down. They're hoping that by releasing Johnson's photo, some of the victims may remember an encounter with the alleged criminal.
Authorities say he targeted women as they were shopping at stores like Target and Kmart, where he once posed as security officer and stopped a woman for shoplifting, telling her he had to search her.
"[He] had her pull her clothing away from her body and was able to take video of her buttocks area with his cellphone," said Hudson.
Detectives say Johnson also filmed women aboard Metrolink and at Union Station. According to authorities, the recordings go back to august 2012.
Johnson is behind bars, held on $750,000 bail. He faces 15 felony counts so far.