A chaotic scene involving Fullerton police officers in October was captured on video by Veth Mam until an officer approached and sent his cellphone flying.
Someone else continued recording as Mam, 35, was thrown to the ground. Mam was arrested and charged with battery, assault on a peace officer and resisting arrest.
Mam's attorney says the officers' reports do not match what's seen on video. Last month, a jury found Mam not guilty.
"It appears we arrested the wrong person in that incident back in October," said Fullerton Police Sgt. Andrew Goodrich.
Police say the admission comes after acting police chief Kevin Hamilton reviewed the incident following accusations of perjury and wrongful arrest. Hamilton has now ordered an internal investigation.
"This incident will be folded into our broader review, which we've been asked to do by the city," said Michael Gennaco, an independent consultant.
Gennaco is conducting a separate review of police department policies following the death of Thomas last month. Thomas' family accuses police of using excessive force as six officers tried to arrest the 37-year-old. The Orange County District Attorney is investigating.
"We know one of these officers was involved in a prior incident," said attorney Garo Mardirossian.
Mardirossian, representing both Thomas and Mam, claims one officer was involved in both altercations. Police won't confirm this, but Gennaco says he'll find out.
"The Kelly Thomas incident is certainly something that's going to be our focus, but other force incidents that come to light, particularly if they involve the same personnel, will be necessary to review," said Gennaco.
Mam's attorney plans to file a federal lawsuit against the Fullerton Police Department.