Months after a protest of the Dakota Access oil pipeline swelled into a movement that drew thousands of people and national attention, most of the remaining demonstrators left the protest camp ahead of deadline Wednesday, leaving a few dozen to face possible arrest.

Even before the 2 p.m. evacuation deadline imposed last week by North Dakota’s governor, the main protest camp had already turned into a muddy pit, the ground soggy with melted snow.

Authorities said 10 people in the area were arrested Wednesday. Between 25 and 50 people were believed to still be at the camp on Wednesday evening, North Dakota Gov. Doug Burgum (R) said during a news conference, though he said this was only an estimate because officials did not know for sure.

The people remaining at the camp who leave voluntarily will be allowed to go, Burgum said. But he said authorities plan to enter the camp Thursday morning for cleanup efforts and that people remaining will be considered trespassing, noting that anyone who interrupts the cleaning could be arrested.

Video footage and images on cable news and social media Wednesday showed flaming structures at the camp that protesters had apparently set on fire in the camp’s final hours.

A policy adviser to the Standing Rock Sioux tribe could not immediately confirm who set the structures ablaze. The Associated Press reported that praying protesters said they were burning the structures, including a yurt and teepee, as part of the ceremony involved in leaving the camp.

Burgum said that during the fires, a 17-year-old woman was severely burned either during a fire that got out of control or an explosion of some kind. She was being airlifted to Minneapolis for medical care, he said. The governor said a 7-year-old boy was also injured. Burgum stressed that officials were not there and “don’t know why … or how it happened.”

The evacuation deadline Wednesday afternoon in North Dakota was the latest confrontation in a bitter fight over a crude-oil pipeline, a standoff on a desolate prairie that drew movie stars, military veterans and investment bankers to an unlikely front line. The tribe has argued that a stretch of the $3.8 billion pipeline threatens its water supply, crosses burial grounds and violates treaties between Native Americans and the federal government.

President Trump has supported the project and ultimately cleared the way for it. He signed an order aimed at expediting the pipeline’s approval before his administration approved final permits needed to complete it. The move came as Standing Rock Sioux Chairman David Archambault II arrived in Washington ahead of a scheduled White House meeting, leading him to say: “I think that I was set up.”

A federal judge has left open the possibility of further court intervention, and the company behind the pipeline has said that oil could flow within the next 30 days.

At the main camp, though, the remaining pipeline protesters — who numbered about 200 or so earlier this month, down from the thousands who gathered in the fall — were preparing to leave the property due to the eviction notice. The Associated Press reported that about 150 of them marched out of the camp about 1 p.m., an hour before the deadline to leave.

The U.S. Army Corps of Engineers issued notices earlier this month saying it was closing federal property in the area that it managed due to “the likely event of flooding in the area,” stating that protest camps were located in a potentially flood-prone spot.

Following that, Burgum signed an executive order last week ordering a mandatory evacuation of people in what he called an “unlawful occupation.” In his order, Burgum wrote that the areas being occupied are in areas that “routinely experience spring flooding and are historically subject to flash flooding,” adding that unseasonably warm temperatures have hastened the potential for flooding.

People camping in the area were given until 2 p.m. Wednesday. The deadline was set to allow contractors “to accelerate the removal of waste from the camp,” Burgum’s office said in a statement.

The Standing Rock Sioux Tribe said Wednesday morning that it was closely monitoring law enforcement activity at the camp and called on “everyone to remain peaceful.”

People who visited and lived at the camps vowed to continue to fight, with one saying this month that despite the decision to allow the pipeline to proceed, “this hasn’t been all for nothing.” Others who had gathered at the camps vowed not to leave, even in the face of the eviction order.

“I’m not going anywhere,” Valerie Armstrong, 36, told CNN. “I carry a knife with me all the time. But I am handing that over so that I have no weapons on me. I will stay and pray, even if they come to remove us.”

One member of the Standing Rock Sioux tribe told The Washington Post that he expected some people would “take a stand both against the pipeline and for indigenous rights,” saying he was concerned that “something may go awry and some people may get hurt.”

Wayne Stenehjem, North Dakota’s attorney general, called on protesters to follow the governor’s order and leave the camp, warning that the area could be “inundated with floodwater.”

“The threat to the environment is very real and the situation is urgent,” Stenehjem said in a statement last week. He added that anyone who remained after the deadline could be arrested, as his office said it was a crime to refuse to comply with an emergency evacuation order.

State authorities said Tuesday that they set up a travel assistance center for protesters leaving the camp. This center will offer snacks, water, hotel lodging and bus fare for a return home, with transportation provided from the camp to the center, according to officials.

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This story has been updated since it was first published.