The bodies of one of the six people who died in the 'Surrey Six' slayings in 2007 is loaded into a coroner's van. CBC

A special prosecutor has been appointed to examine the conduct of four Vancouver-area RCMP officers connected to the investigation of a high-profile gangland killing, which left six people dead.

The appointment follows an outside investigation by the Ontario Provincial Police over a sergeant's alleged relationship with a witness, and brings to five the number of officers connected to the 2007 mass killing who have come under a cloud.

In all, four officers are included in the relationship investigation, while a fifth faces unrelated charges of fraud and attempted fraud in connection with overtime claims. The Crown confirmed the officer facing fraud charges isn't part of the special prosecutor's assignment.

Six people, including two innocent bystanders, were gunned down in a high-rise apartment unit in Surrey in October 2007. Six people have since been charged in the killings, including one who pleaded guilty two years ago.

Officers suspended

Last year, the RCMP revealed the OPP had been brought in to investigate allegations a sergeant involved in the case had an inappropriate relationship with a witness. At the time, the RCMP said that investigation also targeted a co-worker and a supervisor, who weren't involved in the relationship itself.

Now, the Mounties say the number of officers involved in the investigation into the relationship has grown to four, all of whom have been suspended, although the force hasn't said how each are connected to the allegation. The force referred questions to the OPP, which couldn't be reached for comment.

B.C.'s criminal justice branch has appointed Victoria lawyer Christopher Considine as special prosecutor.

The RCMP said in a news release that the four officers targeted by the investigation into the alleged relationship have all been suspended with pay, and they will also face an internal code-of-conduct investigation.

The six people killed included 22-year-old Chris Mohan and 55-year-old gas fitter Ed Schellenberg, both of whom police determined were innocent bystanders.

One man, Dennis Karbovanec, pleaded guilty in April 2009 to three counts of second-degree murder and conspiracy to commit first-degree murder in the slayings.

James Kyle Bacon, Quang Vinh Thang Le, Matthew James Johnston, Cody Rae Haevischer, and Sophon Sek are also charged in the case.