MONTREAL -- An investigation is underway into a violent altercation between several metro officers, a police officer and a Black woman in the Jean-Talon metro this weekend.

A video of the incident from Saturday evening was shared widely on Twitter. It shows a woman screaming repeatedly as she's pinned down by what appear to be at least two metro police officers and then a police officer.

Montreal's transit agency said the incident escalated when the woman bit a transit officer and broke the skin.

One bystander who shot video at the time, Raph K. Yoyo, said he watched as one of the officers "lost his temper" and, in Yoyo's opinion, crossed a line.

"I heard a woman screaming.. a lot of people were gathering," Yoyo told CTV News.

He ran across the station platform and saw two transit agents pinning what he perceived as "an immigrant woman" on the floor.

"They were screaming at her to put her hands behind her back," he said. "She was resisting, but what is not normal is... one of the agents, STM agents, lost his temper and punched the immigrant woman several times on the face."

"This is not okay. Nothing justifies this. This is what I saw," he said.

People who know the woman involved say that she was hospitalized afterwards. A friend of hers told CTV that she can't speak publicly about the incident since they don't yet have her permission to talk to media.

Bystanders said the incident happened around 3:30 on Saturday and wrote online that the woman was punched in the head and put in an attempted chokehold.

On Sunday, some friends of hers organized a letter-writing campaign to City Hall asking for an investigation.

While media have reported the person involved is a man, and Montreal's transit agency referred to her that way, her friends confirmed that this is incorrect and she's a woman.

STM SAYS USE OF FORCE UNDER REVIEW

In a statement, the STM said that the bystander video "does not depict the event in its entirety" and doesn't show the events that led up to the physical struggle.

The person was physically restrained after she fare-dodged and then wouldn't identify herself, the agency wrote.

After a "discussion" lasting several minutes, she "fled, remaining uncooperative," the STM wrote.

"Our inspectors pursued the individual and intervened physically to restrain them," it said. 

The woman "actively resisted" and then, during the struggle, "bit the inspectors several times, causing injuries that required emergency care," the agency wrote.

The woman will be charged with assault causing bodily harm, said the STM.

Montreal police said in a statement that the incident mostly involved transit officers and that "the SPVM intervened in support," directing further questions to the STM.

The STM said Monday that an internal investigation has been launched "to analyze all aspects of this incident, particularly the use of force," which it said was always the case "in this type of intervention."

A spokesperson for the STM, Philippe Dery, later clarified that an internal investigation is standard for all interventions involving the use of force.

"We will be reviewing the circumstances around the event, the chronology, how the intervention of our inspectors was conducted, if the use of force was appropriate for the situation," among other questions, he said."

Transit police are asked to use the same use-of-force standards as the majority of Canadian police forces, the STM said. That model "stipulates that the force applied must be proportional to the degree of resistance and aggression of the individual."

They added that in this case, the woman's resistance was aggressive in that she "bit our employees hard enough to draw blood, notably refusing to release [her] bite."

City Hall hasn't yet responded to a request to clarify if Mayor Plante is also expecting a second investigation external to the STM.

CITY SAYS 'NEUTRAL' INVESTIGATION WILL HAPPEN

On Monday around noon, Montreal Mayor Valerie Plante wrote that some kind of probe is underway at the city, but left unclear if she was referring to the internal probe at the STM or a different one.

"Several people shared an intervention by STM agents that took place this weekend," she wrote, in French, on Twitter.

"These are disturbing images, and an investigation begins today to shed light on what happened."

A spokesperson for Plante's office later suggested she was referring to "the investigation underway," meaning there isn't a separate one planned.

However, the reality turned out to be more complicated. City Councillor Alex Norris said the investigation will be a hybrid one, since Montreal transit officers are only three months away from getting a new status that will subject them to the same oversight as regular police.

"Our intention is to work with the STM to identify a neutral process that will be fully credible [and] that can look into this case and others during the transition period," said Norris, who is a member of Plante's Projet Montreal party and also chairs the city's public safety committee.

The new "constable status" will take effect for STM officers in July.

"This process will result in the possibility of there being external investigations into cases just like this," Norris said, naming the BEI, the provincial police watchdog agency, and the Police Ethics Committee, both of which issue rulings on alleged police misconduct.

"Right now... the [STM] inspectors don't fall under the Police Act," he said. "There's no outside entity that can be called upon to carry out investigations into incidents such as this."

He couldn't give further details on Monday about what the transitional model will look like, but said it's too early to assume the STM will be involved -- despite what the STM earlier said about already launching its own investigation.

He also said he found the video troubling, but and not just for the public, but for STM staff as well.

"One has to remember that there were STM employees who were injured in this incident as well as the individual that they held down," he said. 

"The video is very disturbing. This is not the kind of image that anyone wants to see in our public transit network."

--With files from CTV's Rob Lurie.