A former Dalhousie University medical student convicted of murdering a fellow student is suing the private detective hired to help his defence team, but who instead tipped off Halifax police to incriminating evidence.

In documents filed in Nova Scotia Supreme Court, William Sandeson alleges Bruce Webb, along with his former employer Martin & Associates Investigations Inc. and its owner Tom Martin, failed in their fiduciary duty to him.

Sandeson, 25, was found guilty by a jury last year of first-degree murder in the killing of Taylor Samson, 22, in August 2015. He is serving a life sentence with no parole eligibility for 25 years.

The Crown's theory at trial was that Sandeson set up a drug deal with Samson and then shot him to death in order to steal his nine kilograms of marijuana. Samson's body has never been found.

Taylor Samson is seen on surveillance video outside the apartment of William Sandeson. The footage was played at Sandeson's trial. (CBC)

Two of Sandeson's neighbours, Justin Blades and Pookiel McCabe, testified at trial about hearing a loud noise coming from Sandeson's Halifax apartment the night Samson disappeared.

The pair got a glimpse inside the apartment moments later. They told the court about seeing a man slumped over at the kitchen table, a pool of blood forming at his feet and money scattered on the floor.

The dramatic testimony was one of the key moments of the trial. But Sandeson argues it should never have happened.

Samson, 22, was reported missing on Aug. 16, 2015. His body has never been found. (Halifax Regional Police)

In their initial statement to police, both men had claimed they saw and heard nothing that night.

In preparation for trial, Sandeson's lawyer, Eugene Tan, hired Martin & Associates to interview prospective witnesses. Martin dispatched Webb, a retired RCMP officer, to track down Blades and McCabe.

While they'd withheld information from police, they opened up to Webb. The pair said they had been afraid to tell the truth because they believed Sandeson had ties to the Hells Angels.

Webb in turn alerted a neighbour, Halifax Regional Police Supt. Richard Lane, that Blades and McCabe had changed their stories. Webb also facilitated a meeting between the men and police investigators.

Sandeson only found out in the middle of his trial what Webb had done. His lawyers asked for a mistrial, and Webb was brought in to testify at a hearing without the jury in the courtroom.

"It appeared that Mr. Sandeson's case wasn't really good for him," Webb testified. "I also felt that at that point, if I didn't come forward, I would be obstructing justice."

Justice Josh Arnold denied the defence request for a mistrial.

"While there is a reasonable possibility that the involvement of Webb has some relevance, the materiality of Webb's involvement with the police in obtaining statements from Blades and McCabe is relatively low," Arnold wrote in refusing to grant the mistrial.

Arnold described the possible damage to Sandeson's fair trial rights caused by Webb's actions as "insignificant."

The mistrial application was one of several diversions that cropped up in the course of the murder trial. The jury was sent out of the courtroom for days while lawyers argued over whether Webb's actions breached Sandeson's solicitor-client privileges. The judge ruled they did not.

Important to case

However, in his civil lawsuit Sandeson claims it was a breach.

In seeking unspecified damages from Martin, Webb and the detective agency, Sandeson alleges: "Negligence, breach of contract, breach of litigation privilege and/or breach of duty owed to the Plaintiff."

The lawsuit, filed by lawyer Stacey England, alleges that "Webb's involvement ensured [Sandeson] was prosecuted and found guilty of the charges against him."

England writes that Sandeson "would have had a reasonable prospect of success at trial of being acquitted had Webb not conducted himself in the manner described herein."

While Blades and McCabe provided dramatic moments in the trial, the Crown had amassed a great deal of other evidence against Sandeson. It included video surveillance footage, text messages, and DNA that showed Samson's blood was inside Sandeson's apartment, his car and in objects recovered from the Sandeson family farm near Truro.

Martin has no comment

None of the allegations contained in the lawsuit has been tested in court.

Martin, a former Halifax Regional Police homicide investigator, said he would not comment on the lawsuit, on the advice of his lawyer. He told CBC News his lawyer will be filing a defence in the coming weeks.

Webb couldn't be reached for comment.

Sandeson is appealing his murder conviction. He's also used his time in prison to launch a successful small claims court action against his former roommate and make a bid to recover the laptop computer police seized from his apartment.

No date has been set for his murder appeal.