Metro Vancouver transit police are asking an unidentified woman to come forward, after a known sex offender making duck noises allegedly sexually assaulted her on the SkyTrain in April.

Police identified the alleged attacker after a fellow passenger called police around 11:00 a.m. PT on April 20 to say she had witnessed a man assaulting a woman after they boarded at Commercial-Broadway station.

According to the passenger, the suspect allegedly sat next to the woman and began touching and groping her, despite being asked to stop several times, and made loud quacking sounds at her and other passengers.

In a statement, spokesperson Anne Drennan said police quickly identified the suspect but have no way of contacting the witness or identifying the victim.

"The witness, who provided suspect and victim descriptions did not wish to provide her name and contact information at that time," said Drennan.

"[We] are appealing for the victim and witnesses to come forward to assist with the investigation."

The victim is described as a woman of Aboriginal or Asian origin, aged in her early 20s, with shoulder-length black hair in a bun. It is thought she had a shopping style cart with her. 

The 49-year-old suspect, a known sex offender already identified by police, is white, bald, and has a long grey goatee.

According to the witness, he was wearing a black long-sleeved shirt, with a large cross and the word "chopper" on the back. He was also wearing a black brace on his left knee and walked with a limp. 

Anyone with information about the incident is asked to call Transit Police on 604-515-8300 or text 87-77-77.