Marc Parent said Tuesday the 33-year veteran of the force worked in the intelligence unit and was one of a handful of people who had access to a confidential list of names.
Several reports said he allegedly had contact with a lawyer who represents organized crime figures and that he offered information in exchange for a six-figure sum.
He allegedly gave the lawyer four names to show he was serious — people who have since been contacted by police, Parent said.
The man has not been charged and his name has not been made public. The alleged offences occurred after his retirement in January 2011.
Police say the information that led to the allegations surfaced in April 2011 during a wiretap investigation.
An ongoing internal investigation revealed that none of the informants was deemed to be at risk, Parent said. The force is now reviewing computer security and protocols for having access to sensitive information.
"In this situation we have an individual who, in bad faith and dishonestly, decided to share information," Parent said, flanked by the mayor and the city councillor in charge of public security.
"We must put in place a system that allows us to detect them and to have a rigorous management of these units."
The former officer, a detective-sergeant with an exemplary track record, was arrested and released last October.
The police chief was clear about what he thought about an officer — retired or working — acting in such a fashion.
"If a police officer or a former officer does something similar to what we're referring to, I consider this (person) has done something unacceptable," Parent said.
"I would like to reassure the population that we will do everything to shed light on this matter."
Mayor Gerald Tremblay said he was aware of the investigation but was limited in the comments he could make.
"The police department has my full confidence," Tremblay said in a brief statement.
"You have to remember there are more than 7,000 people working with the department and if there's a rotten apple that does things that should not be done, we have a responsibility to clear the situation as soon as possible."