Stephen Collins -- L.A. County Sheriff Investigates Sex Crime ... Victim Comes Forward

Stephen Collins is now under investigation by a third law enforcement agency for allegedly molesting and/or exposing himself to young girls ... TMZ has confirmed ... and the victim herself is the one who came forward to make the complaint.

Law enforcement sources tell TMZ ... the woman was 13 at the time and she has now gone to law investigators with her allegations.

Our sources say ... the incident involves a relative of a neighbor when Collins lived on Havenhurst Drive in West Hollywood during the summer of 1983.

Collins confesses to exposing himself to the girl on the audio tape posted on TMZ last week. We redacted her name and address.

Collins also says on the audio ... he went back to the victim and "made amends" years later.

We're told the New York PD referred the matter to the L.A. County Sheriff's Dept. because the incident occurred in that jurisdiction.

We're told the Sheriff's Dept. investigation centers around indecent exposure and the crime of annoying or molesting a child. We've learned Collins is listed as "suspect."