Officers claim protesters set off explosion but Sophia Wilansky’s father and on-site medical professionals say injury is ‘entirely consistent’ with grenade blast

North Dakota law enforcement have blamed protesters for a woman’s grievous injury at the Dakota Access pipeline protests on Sunday, despite assertions from the woman’s father and a witness to the incident that she was hit by a police concussion grenade.

Sophia Wilansky, an environmental activist from New York, was hospitalized in Minneapolis where surgeons are attempting to repair a severe injury to her left arm that destroyed arteries, nerves, muscle, soft tissue and bone, according to her father. She remains at risk of amputation, and if the arm is salvaged, it will probably have very limited functionality.

Wayne Wilansky, her father, contends that the injury was caused by an exploding concussion grenade thrown by law enforcement, who also deployed teargas, rubber bullets and a water cannon on protesters during a tense standoff on a bridge Sunday night.

But North Dakota law enforcement officers have aggressively countered Wilansky’s account, releasing multiple statements accusing protesters of setting off an explosion.

Lieutenant Tom Iverson of the North Dakota highway patrol said that two men and a woman were standing near a burned vehicle on the bridge around 3am when “it became obvious that they were tampering with the vehicle or planting a device”. The highway patrol said the explosion occurred after other protesters rolled “metallic cylinder objects” toward the three near the car, and that the female protester was pulled from under the vehicle before the group “fled the scene”.

The highway patrol also released photographs of propane cylinders recovered from the scene of the confrontation, which they said were being used as explosives.

Wayne Wilansky, who was not in North Dakota at the time of his daughter’s injury, spoke to the Guardian from the hospital and conveyed his daughter’s account. He maintains that the police account was “completely fictitious” and “bogus nonsense”.

“She wasn’t near the truck at the time it happened and was backing away while being shot with rubber bullets,” he said. “While backing away and trying not to be hit with the bullets the grenade was thrown directly at her and it exploded on her arm.”

A witness, Stephen Joachinson, told the Guardian that he was talking with Sophia Wilansky near the burned vehicle when police started yelling at them on a bullhorn.

“They threatened that they were going to shoot us with less-than-lethal force and within 20 seconds of that warning they just started firing on us,” he said.

Standing Rock protests The Dakota Access pipeline (DAPL) is a $3.7bn project that would transport crude oil from the Bakken oil field in North Dakota to a refinery in Patoka, Illinois, near Chicago. The 1,172-mile pipeline would carry 470,000 barrels a day.

The local Standing Rock Sioux tribe and thousands of Native American supporters say the project threatens sacred native lands and could contaminate their water supply from the Missouri river.

Police, who are often armed with large tanks and riot gear, have used pepper spray, teargas, rubber bullets, Tasers and other “less-than-lethal” tools against protesters.

The pair began running away, he said, when a concussion grenade exploded behind them. Seconds later, a second concussion grenade struck her arm.

“When she got hit with the rubber bullet she was hurt and she was scared and she ran back, and that’s when they hit her with the concussion grenade and it blew up right on her arm,” Joachinson said.

A spokeswoman for the Morton County sheriff’s department told the Los Angeles Times that officers did not deploy any concussion grenades on Sunday. Officials have said that they deployed teargas, bean-bag rounds, sponge rounds and a water cannon.

Law enforcement officials did not respond to queries from the Guardian seeking a list of all the types of “less-than-lethal” weapons used on Sunday, a list of all the law enforcement agencies present and further details on its statements about the incident.

“The injuries are inconsistent with any resources utilized by law enforcement and are not a direct result of any tools or weapons used by law enforcement,” Iverson said in a statement.

Wayne Wilansky called law enforcement’s denial of using concussion grenades “nonsensical”.

“The doctor pulled shrapnel out of her arm,” he said, adding that surgeons had informed him that the injury was “entirely consistent” with a grenade explosion.

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The Standing Rock Healer and Medical Council, a group of medical professionals at the encampments, also rejected the claim that the injury was caused by a propane explosion, citing witness accounts, the “lack of charring of flesh at the wound site” and “grenade pieces that have been removed from her arm in surgery”.

The explosion is under investigation by the North Dakota bureau of criminal investigation with support from the Federal Bureau of Alcohol, Tobacco, Firearms and Explosives. Wilansky said that FBI officers had visited the hospital and collected his daughter’s clothing on Tuesday. A spokesman for the FBI refused to confirm or deny that the agency was involved in the investigation.

Sophia Wilansky is one of thousands of activists who have travelled to the banks of the Missouri river near the Standing Rock Sioux reservation in North Dakota to support the tribe’s efforts to halt the Dakota Access pipeline. Members of the tribe fear that the pipeline – which is slated to cross under the river – will contaminate their water supply and object to construction on land that includes sacred burial sites.

The movement has become a global rallying cry for indigenous rights and anti-climate change activism, but has faced fierce opposition from the pipeline company and local law enforcement. Morton County’s sheriff ,Kyle Kirchmeier, said in a statement on Monday that the tribe had been “hijacked” by “violent factions” made up of “evil agitators”.

More than 500 people have been arrested during the months-long standoff, and law enforcement officials have deployed pepper spray, teargas, rubber bullets, Tasers, sound weapons and other “less-than-lethal” methods.

The Mandan police chief, Jason Ziegler, said in a press conference on Monday that law enforcement agencies “can use whatever force necessary to maintain peace”.

Sara Lafleur-Vetter contributed reporting