A 21-year-old woman from the Bronx may lose her arm after being hit with a concussion grenade during a police assault on protesters at the Standing Rock Sioux Reservation in North Dakota Sunday night.

According to the Daily News, the explosion from the grenade "tore through" the winter coat and skin of activist Sophia Wilansky, leaving bone exposed. Photos of Wilansky sitting in a vehicle with a gaping wound and visible broken bones have circulated online.

Wilansky is one of thousands of activists from around the country who have traveled to Standing Rock to protest the building of the pipeline, which would pass through sacred tribal burial grounds and, the tribe says, threaten the local water supply.

Several hundred protestors were involved in the clash with local law enforcement, which left over 300 injured, 26 hospitalized

, and also led to 16 arrests, according to medics from the Standing Rock Medic and Healer Council.

According to protesters, the confrontation began when they attempted to remove two charred military trucks from a bridge north of a protestor encampment. They claimed the trucks were preventing emergency services from reaching the camp. In temperatures below freezing, the police fired upon protestors with water cannons, as well as rubber bullets, sonic weapons, tear gas, and flash bangs, Telesur reported.

According to a GoFundMe page that was set up to raise money for Wilanksy's medical bills, she was giving out bottles of water on the front lines when she was hit with the projectile that shattered her arm.

A spokesperson for the Morton County, N.D. Sheriff's Department told the LA Times that police had not deployed concussion grenades against protesters. She suggested Wilansky may have been injured by explosives rigged up by the protesters themselves.

On Monday morning, Wilansky was airlifted 400 miles to the Hennepin County Medical Center in Minneapolis, where she underwent eight hours of surgery. Her father, Wayne Wilansky, told the Guardian that his daughter will require several days of treatment. He said that doctors are not yet sure whether they will need to amputate, but that he believes police threw the concussion grenade at his daughter.