The May 20, 2012 incident, which took place at the corner of Ste-Catherine and St-Hubert Streets, was recorded on a cellphone video that went viral online, with nearly half a million hits on YouTube.
Quebec's Director of Criminal and Penal Prosecutions (DCPP) reviewed the evidence and concluded the video didn't allow prosecutors to "appreciate the entire event in context."
The DCPP also said the student demonstration had already been declared illegal at the time of the incident, and that Trudeau's actions didn't exceed limits set out in Article 25 of the Criminal Code (which lays down the ground rules for the use of "necessary force" by an officer of the law.)
After the incident went viral, Trudeau was removed from "student protest" duty pending an internal inquiry, but avoided suspension. The police ethics commission also launched an investigation.
A few months later, in October 2012, Trudeau was once again accused of using excessive force, after the internet release of a cellphone video showing Trudeau and at least two other officers involved in a forceful confrontation in a stairwell on Papineau Avenue. Trudeau was seen screaming and putting one man in a headlock.
The three men and one woman involved say the use of force was unprovoked and that they had no idea what was going on or why they were being detained. Charges against the the three men for assaulting a peace officer and obstructing police were later dropped.
Trudeau was suspended from duty after that second incident, and Montreal Police Chief Marc Parent apologized to the public for her behaviour. He called it "unacceptable" and saying Trudeau "absolutely" posed a danger to the public.
Trudeau could still face sanctions arising from that incident, ranging from a reprimand to dismissal.