Two teenage girls face murder charges after 19-year-old woman, Serena McKay, died in what RCMP say was a homicide in the Sagkeeng First Nation northeast of Winnipeg.

Police confirmed on Tuesday McKay's death was a homicide, having previously characterized it as a suspicious death.

The girls, age 16 and 17, have been arrested on second-degree murder charges and are in custody, say RCMP.

McKay and both of the accused attended Sagkeeng Anicinabe High School, said principal Claude Guimond, who added the victim was set to graduate this year.

Guimond said a short video that has been circulating on Facebook in recent days shows what he described as a violent assault on McKay. He said he believes drugs were involved in the attack.

"After seeing what I saw on the video, you know what? There's nobody in their right mind [that] would do something like that, unless they were extremely high on whatever and just totally, like, out of it," he told CBC News.

RCMP say they're aware of the video, but a spokesman would not confirm if the person shown being attacked was in fact McKay.

Sgt. Paul Manaigre said officers are reviewing the video to determine if and how it forms part of their investigation.

"Posting videos to the internet is not a crime," he said. "If someone was there and actively involved [it] is a whole different aspect that will be investigated as part of this matter."

The fact that such violent footage is being shared by people on social media is shocking, Manaigre added.

"I can't imagine what the family, the community is going through right now. This is a 19-year-old female; this is someone's child, cousin, niece, sister. They're in shock, they're grieving over her loss and they're trying to come to terms with it," he said.

"The internet, being what it is today, we have to be careful ... what we do. Everything is being put out there. It can be dangerous."

Classmates 'almost speechless'

Sagkeeng Chief Derrick Henderson said McKay was not from Sagkeeng but was going to school there. Police confirmed she was living in the neighbouring community of Powerview-Pine Falls, Man.

The neighbouring communities are about 100 kilometres northeast of Winnipeg.

Classmates expressed shock and disbelief at McKay's death during a healing ceremony at the school Tuesday morning, Guimond said.

"They were shocked to the core that this could actually happen, that they interacted with these students on a daily basis and they couldn't believe that there could be something as horrific as this," he said.

"It left them almost speechless."

Guimond said grief counsellors are on hand for the rest of this week, available to speak with students.

Last seen Saturday evening

The victim was last seen by a family friend on Saturday evening and was reported missing to Powerview RCMP on Sunday around 6 p.m., say police.

As officers searched the area, they received a call two hours later — around 8 p.m. — that the body of a woman was found near a home in the First Nation.

Henderson said police were in the community all day Monday.

RCMP say investigators from its major crime and forensic identification units remain at the scene on Tuesday.

"Oh, very bad, very, very. It's very sad," Henderson said.

A vigil is being planned for Thursday evening. Guimond said the school will hold a memorial for McKay at some point as well.