The late-night arrest on Tuesday of Toronto Mayor Rob Ford's friend and occasional driver is part of a broader police investigation that was launched after detectives came across unexpected information during a different probe of a drug-dealing street gang, The Globe and Mail has learned.

Alessandro Lisi, a 35-year-old who has no official role in the mayor's office but sometimes chauffeured the mayor to events, was charged Wednesday with four criminal offences, including marijuana trafficking. But the arrest of Mr. Lisi – whom Mr. Ford called a "good guy" and "on the straight and narrow" at a news conference Wednesday – is part of a probe that is much more expansive.

Toronto police have placed at the helm of this investigation one of the force's most seasoned detectives, Detective Sergeant Gary Giroux of the homicide squad. Along with another prominent city investigator, Detective Joyce Schertzer, he is heading a team examining at least two individuals close to the mayor as part of that probe, according to several sources with knowledge of the police inquiries.

Story continues below advertisement

Det. Sgt. Giroux's probe stems from Project Traveller, a year-long investigation that culminated in the June raids of a series of west-end Toronto high-rise apartments that police say served as the base for a drug-dealing gang known as the Dixon City Bloods. But what started as an investigation in one of the poorer neighbourhoods in the Toronto suburb of Etobicoke has extended into the area's more affluent streets – and the mayor's office. Five former staffers from Mr. Ford's office have been interviewed by police, several sources have told The Globe, and one of the areas of interest for investigators, the sources say, is Mr. Lisi.

At a news conference at a gas station near his house, Mr. Ford declined to answer questions about how he knows Mr. Lisi. He called Mr. Lisi "a friend."

"He's a good guy. I don't throw my friends under the bus," he said before departing for a trip to Austin, Tex. Later, when asked whether he's worried about police surveillance on Mr. Lisi potentially picking up his communication with him, the mayor shook his head and remained silent.

Mr. Lisi, who was charged with trafficking in marijuana, possession of marijuana, possession of proceeds of crime and conspiracy to commit an indictable offence, was granted bail Wednesday afternoon. After departing Toronto's Old City Hall courthouse, he did not respond to questions from reporters about his relationship with the mayor as he weaved through the Eaton Centre, an office building and a pub, before getting into a cab. Mr. Lisi's lawyer, Courtney Keystone, declined comment.

Exactly how Mr. Ford and Mr. Lisi, who is more than 10 years younger than the mayor, became acquainted is unclear. He became a fixture by the mayor's side over the past year, sources close to the mayor's office said. On the morning of May 17, after the Toronto Star and the U.S. gossip website Gawker revealed that drug dealers had tried to sell them an alleged video of the mayor purportedly smoking crack cocaine, he was captured on news cameras at a gas station near the mayor's house, where he stood between a pack of reporters and the mayor.

When the marijuana-related charges against Mr. Lisi were announced in a press release Wednesday, it identified Det. Sgt. Giroux as the lead investigator – an official communication that left many in Toronto's legal circles wondering why such a high-profile detective would be involved in a marijuana arrest, something considered less serious.

Mark Pugash, a spokesman for Toronto police, said it was not uncommon for the force to use Det. Sgt. Giroux in investigations other than murders. He pointed to the officer's role in examining crimes committed during the G20 summit in 2010. He called him an "extremely experienced, extremely able and extremely tenacious investigator."

Story continues below advertisement

"I think when the documents are revealed in court, people might have a better understanding of what's involved," Mr. Pugash added. "There's nothing else I can say now because it's before the courts, but as information becomes available it will explain the context in which [Det. Sgt. Giroux] worked."