The American Civil Liberties Union and Amnesty International are stepping up calls for the Department of Justice  to investigate what they feel is excessive use of force by law enforcement in North Dakota in response to protests against the Dakota Access pipeline.

The renewed calls for action come after police used tactics Sunday night that included the use of rubber bullets, concussion grenades, tear gas and water cannons sprayed on hundreds of protesters for hours in freezing temperatures, according to live streamed video and protesters’ accounts.

At least 300 individuals received medical treatment during Sunday’s standoff. One protester, Sophia Wilansky who traveled to Cannon Ball, N.D. from New York, faces potential amputation after an explosion severely damaged her left arm.

“Law enforcement shouldn’t treat protesters like an enemy on the battlefield and that unfortunately is what we saw Sunday night,” Amnesty International USA spokesman Eric Ferrero said. ”We have very serious concerns about law enforcement violating the human right of protesters.”

Amnesty sent a letter to U.S. Attorney General Loretta Lynch on Oct. 28 calling for the Department of Justice to investigate the actions of law enforcement agencies.

The letter cites the repeated use of riot gear and  pointing weapons, including assault rifles, at protesters with no signs of  imminent threat to the officers or others. It also criticizes the arrest of members of the media and legal observers. The letter stated that law enforcement erected roadblocks on roads leading to and from the protest camps, that individuals arrested were strip-searched and required to post bail, even for minor offenses such as trespassing, and that officers used pepper spray “indiscriminately against protesters without provocation.”

On Monday, Amnesty sent a letter to Morton County Sheriff Kyle Kirchmeier decrying the use of excessive force during Sunday night’s confrontation with protesters.

“Policing and security equipment—such as rubber bullets, tear gas and stun grenades, often described as “less-lethal” weapons—can result in serious injury and even death,” the letter stated.

Similar concerns were voiced by the American Civil Liberties Union.

“When you have hundreds of people enclosed in a space on the bridge and you are using water cannons combined with chemical agents and other type of less lethal weapons, the scenario for that is just unjustifiable,” said Jennifer Cook, the ACLU’s North Dakota policy director.

The ACLU sent a letter to the Justice Department on Nov. 4 calling for an investigation of possible “constitutional and federal rights violations” in the police response to Dakota Access pipeline protesters.

Cook said the current escalation is similar to that seen in Ferguson, Mo. in 2014 following the shooting of unarmed Michael Brown by a police officer.  The Justice Department did investigate police behavior in that case and the Ferguson police chief resigned following the investigation. Five other members of the department resigned or were fired.

“The DOJ eventually found that police tactics certainly escalated the tensions in Ferguson, Mo. and helped lead the way to riots,” Cook said. “We’ve asked the DOJ to ascertain whether the police tactics that have been used against DAPL protesters have led to the escalation in tensions.”

Morton County officials said the law enforcement response to protesters on Sunday was appropriate.  

“In such circumstances, the use of any tool, method or weapon of opportunity must nonetheless be objectively reasonable and utilized only to the degree reasonably necessary to accomplish a legitimate law enforcement purpose,” Morton County sheriff’s spokeswoman Maxine Herr said in an email. “Water hoses were used to keep distance between officers and the criminal agitators, and to put out fires set by the agitators.  It’s an alternative to less lethal and chemical munitions.”

“We are aware of the information about the woman on social media who has claimed she sustained injuries to her arm due to law enforcement tactics,” North Dakota Highway Patrol Lieutenant Tom Iverson said in a statement. “The injuries sustained are inconsistent with any resources utilized by law enforcement and are not a direct result of any tools or weapons used by law enforcement.”

The Morton County Sheriff’s Department posted a photo on its Facebook page on Wednesday of a green propane tank it said was found at the protest site following an explosion early Monday morning. “Officers witnessed protesters rolling the cylinder on the bridge, saw the explosion and then witnessed protesters running on the bridge to carry a woman from the scene,” the post stated.

Herr said law enforcement personnel did not use concussion grenades but they do have one report of a “stinger ball” being used during the protest. “The device is less than lethal and is used as a distraction device,” she said. “It makes a loud noise and emits small rubber balls which [are] meant to cause people to be startled by it and therefore disperse. It contains no shrapnel.”

After a Sept. 3 confrontation between Dakota Access security personnel with dogs clashed with protesters, North Dakota Rep. Kevin Cramer called for increased law enforcement. “Only an overwhelming law enforcement presence will stem the tide of future lawlessness,” Cramer wrote in letters to the FBI, U.S. Marshals, Bureau of Tobacco, Firearms and Explosives, Bureau of Indian Affairs and the National Park Service.

Representative Cramer referred to protesters involved in Sunday’s standoff as “criminals” and “paid rioters” who “attacked law enforcement.”

“What evidence do they have that there was excessive force?” Cramer asked. “In North Dakota we are not as confused about the difference between breaking laws and enforcing laws as they are in other parts of the country.”

Cramer received $187,500 from the oil and gas industry, more than from any other industry, during his successful 2016 re-election campaign. That money exceeded the total of all campaign funding received by his opponent, Chase Iron Eyes, a member of the Standing Rock Sioux, the tribe leading the opposition to the pipeline.

Cramer, an early energy adviser to President-elect Donald Trump, said he has not discussed the use of force with the Morton County Sheriff’s department other than to “tell them that I am trying to get them more resources from the federal government.”

Cramer along with North Dakota Gov. Jack Dalrymple and Sen. John Hoeven, all Republicans, urged President Obama to provide federal law enforcement in a letter sent on Wednesday.

ICN’s Zahra Hirji contributed reporting for this story.