Activists protesting the Dakota Access oil pipeline said a demonstrator was undergoing multiple rounds of emergency surgery Monday after an explosion severely injured her left arm during clashes with law enforcement near the project’s planned construction sites.

Anti-pipeline activists accused authorities of firing a concussion grenade that tore open the woman’s arm while she was distributing water Sunday night, while police said officers on the scene in Cannon Ball, N.D., did not deploy anything that would have caused such damage.

The conflicting accounts came after an hours-long confrontation between heavily armed law enforcement and roughly 400 protesters trying to move past a barricaded bridge about a mile south from where pipeline drilling is set to take place. With temperatures in the mid-20s, authorities sprayed the crowd with tear gas and water from hoses, and protesters reported being pelted with rubber bullets and concussion grenades.

The Standing Rock Medic and Healer Council said in a statement that its medical teams treated 300 people for injuries that were a “direct result of excessive force by police.” At least 26 people were transported from the scene by ambulance with gashes, internal bleeding and eye trauma, the council said.

The Morton County Sheriff’s Department has defended officers’ use of force, saying hoses and gas were necessary to control a “very aggressive” crowd. One officer was injured and one person was arrested during the confrontation, which started around 6 p.m. Sunday and lasted into Monday morning.

Pictures posted on social media and emailed to The Washington Post showed a woman in a green jacket with her eyes closed sitting in the front seat of a car and supporting her left arm. Close-up images show exposed bone, muscle and blood.

Activists, who call themselves “water protectors,” identified the injured woman as Sophia Wilansky and said she had traveled to North Dakota from New York several weeks ago to participate in the demonstrations.

Michael Basillas, who said he was friends with Wilansky, posted a video to Facebook late Monday saying she was airlifted to Hennepin County Medical Center in Minneapolis, where she underwent a round of surgery. She was scheduled to go in for a second round in the next day, he said. Initially, rumor spread among some activists that Wilansky was going to have her arm amputated.

Basillas, who is based in New York, and protesters in North Dakota said Wilansky was handing out water to people on the “front lines” of the protest when she was hit by a concussion grenade authorities fired in her direction. Police have denied the allegation, saying protesters were responsible, but Basillas insisted otherwise.

“The violence did come from the police,” he said. “And we are prepared to hold them accountable for their action.”

A public information officer for the Morton County Sheriff’s Department was not immediately available for comment Monday night. Reached by phone, a dispatcher for the department told The Washington Post that police only used nonlethal weapons against the protesters.

“Nothing that was used was strong enough to injure anyone significantly,” said the dispatcher, who declined to give her name.

In a news conference Monday, Sheriff Kyle Kirchmeier said there was an “explosion” in the area where protesters were gathered, but it did not come from police, who he said used hoses and pepper spray to disperse the crowd.

“We don’t know where it came from, but it wasn’t law enforcement,” Kirchmeier said.

A spokeswoman for the department, Maxine Herr, told the Los Angeles Times that police did not use any concussion grenades in Sunday night’s standoff. She added that medics first encountered the injured woman at a casino near the protests.

“It wasn’t from our law enforcement, because we didn’t deploy anything that should have caused that type of damage to her arm,” Herr said. “We’re not sure how her injury was sustained.”

According to the Times, Herr suggested that protesters may have been “rigging up” explosive propane bottles to be thrown at police. Kirchmeier said “green bottle Colemans” — a brand of commercial propane cylinders — were thrown at officers.

A Facebook Live video posted Sunday night by activist Kevin Gilbertt showed dozens of protesters facing off against a line of armored vehicles and police in riot gear near the Backwater Bridge on Highway 1806, not far from a camp site that activists have used a staging ground for demonstrations. The bridge has been blocked by burned-out trucks, wire and concrete barriers since late October, when activists tried to set up a second protest camp on nearby private property.

At one point in Gilbertt’s video, a boom is heard and sparks can be seen flying into the air. Gilbertt can be heard saying he believed that police had launched a concussion grenade. In a follow-up video discussing Wilansky’s injury, he disputed claims that protesters had used any explosive devices.

“I have no words for that,” he said.

A GoFundMe page launched to cover Wilansky’s treatment had raised more than $120,000 from roughly 4,300 donors as of Tuesday morning, far surpassing its initial goal of $20,000. Activists say they will hold a news conference outside the hospital at noon. Vigils are planned in Minneapolis and New York in the coming days.

Native American tribes and activists from across the country have gathered at the Standing Rock Indian Reservation for months to demonstrate against the Dakota Access pipeline, which is slated to carry oil nearly 1,200 miles from North Dakota to Illinois. The Standing Rock Sioux argue the pipeline’s planned path under the Missouri River threatens their water supply and cultural sites. Project developer Energy Transfer Partners and North Dakota regulators contend the pipeline is environmentally sound and will not disrupt Sioux sacred lands.

Sunday’s clashes between authorities and protesters started in the early evening, when some protesters tried to move trucks blocking the Backwater Bridge, saying they were preventing emergency services from accessing the reservation. Authorities say they have left the bridge closed since late October because they are concerned about its structural integrity. Several hundred people have been arrested in connection with the protests since the summer.

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