A Toronto police officer is expected to be charged with two counts of professional misconduct in connection with the case of serial killer Bruce McArthur.

Sgt. Paul Gauthier is set to appear at a tribunal Tuesday on charges of insubordination and neglect of duty under the Police Services Act, his lawyer Lawrence Gridin tells CBC News. The allegations are not criminal in nature and have not been tested in court.

On Tuesday, McArthur, 67, pleaded guilty to killing eight men, many of whom had ties to Toronto's Gay Village, between 2010 and 2017.

But the allegations against Gauthier relate to an early interaction between McArthur and police, which is regarded by some as a missed opportunity.

In 2017, Chief Mark Saunders publicly dismissed the idea of a serial killer in the Village, remarks that drew the ire of residents once McArthur was arrested.

But a police source close to the investigation previously told CBC News officers spoke to McArthur as part of an investigation not connected with the broader investigations into disappearances in the Village.

McArthur's monstrous nature was difficult to uncover because he led a life of extreme deception. - Lawrence Gridin, lawyer for Sgt. Paul Gauthier

Reports emerged that a man had once told police McArthur had tried to strangle him. Police questioned, then released McArthur sometime before 2017, a move that later prompted the Toronto police professional standards unit to launch an internal investigation into the matter.

In a statement Friday evening, Gridin said, "The decision not to charge Bruce McArthur for the 2016 incident was made in conjunction with Detective Gauthier's supervisor and based on the information available at the time."

At least three of McArthur's victims are believed to have been killed after 2016. His victims were Skandaraj Navaratnam, 40, Andrew Kinsman, 49, Selim Esen, 44, and Abdulbasir Faizi, 44, Kirushna Kumar Kanagaratnam, 37, Dean Lisowick, 47, Soroush Mahmudi, 50, and Majeed Kayhan, 58.

McArthur pleaded guilty to killing these eight men. Top row, from left to right, Skandaraj Navaratnam, 40, Andrew Kinsman, 49, Selim Esen, 44, and Abdulbasir Faizi, 44. Bottom row, from left to right: Kirushna Kumar Kanagaratnam, 37, Dean Lisowick, 47, Soroush Mahmudi, 50, and Majeed Kayhan, 58. (Toronto Police Service/CBC)

Police believe Lisowick was killed sometime between 2016 and 2017. Unlike most of McArthur's other victims, Lisowick was never reported missing.

Esen disappeared from area of Yonge and Bloor streets over the Easter weekend in 2017. He was reported missing on April 30, never to be seen again.

Just two months later, Kinsman vanished from Toronto's Cabbagetown neighbourhood, one day after the annual Pride parade. He was reported missing three days later.

Further details about the allegations against Gauthier are expected after his appearance Tuesday.

The statement from his lawyer went on to say Gauthier conducted a "proper" investigation of McArthur, and made the information available to all other investigators involved.

"McArthur's monstrous nature was difficult to uncover because he led a life of extreme deception," the statement said. "Det. Gauthier has great sympathy for the victims and the community."

How the Toronto Police Service has handled missing persons cases is now the subject of an independent review led by former Ontario Court of Appeal judge Gloria Epstein.

In a statement late Friday night, Toronto Police said homicide investigators immediately contacted the professional standards unit when investigative concerns were identified.

"As a result, an officer has been compelled to attend a tribunal in efforts to provide an explanation for his actions."

The statement went on to point out the role that community members played in solving the cases of the missing.

"The success of the McArthur investigation was a result of the community working with the police."