According to police, the manager, Stephen Sharp, urinated in the mix at least twice for his own alleged "sexual gratification”. An estimated 30 to 40 people may have consumed those milkshakes.
Mr Sharp was arrested by Vancouver police and reportedly confessed to downloading and distributing child sex abuse images. During that confession, police claim he also admitted to having a "sexual interest in children”.
Police then conducted a search warrant and found the man allegedly had dozens of photos and videos of child sex abuse images on his digital devices. Among the child sex abuse images were also allegedly videos of him urinating in the milkshake mix at Arby's.
Mr Sharp told investigators he was "almost sure" he threw the bag of shake mix away after he urinated into it, but it remains unclear. He told detectives that if he did not throw the bag away, then it was almost certainly used to make shakes for the restaurant.
Investigators said they did not believe that any of the other workers knew about their manager's bizarre fetish.
Mr Sharp was ultimately charged with four counts of possessing child sex abuse images and four counts of dealing those images.
Law enforcement is looking for anyone who ordered a milkshake from the 221 Northeast 104th Avenue Arby's in Vancouver on 30 October or 31 October — and who has a receipt — to contact them.
“The Digital Evidence Cybercrime Unit has not located evidence that Arby’s or its franchisee was aware of the abhorrent actions of Stephen Sharp,” Vancouver police Sgt. Julie Ballou told The Columbian.
Arby's representatives have condemned Mr Sharp's alleged actions.
"The alleged actions of the former franchised employee are abhorrent and unacceptable," an Arby's spokesperson said in a statement to FOX broadcasters. "We have taken immediate action with the franchise group who terminated the former employee and is fully cooperating with the authorities in their investigation."
Mr Sharp is currently booked at the Clark County Jail. On Friday, his bail was increased from $5,000 to $40,000, court records show.