Winter has arrived in Standing Rock at the Oceti Sakowin Camp in North Dakota, the day after the Army Corps of Engineers denied the easement needed to build the pipeline.

A civilian leader in the Army made the decision to deny an easement to the controversial Dakota Access oil pipeline despite Army Corps of Engineers recommendations that it be granted, according to officials and a document.

But because of the pipeline's size — 30 inches in diameter — its approval went to Assistant Secretary of the Army for Civil Works Jo-Ellen Darcy, an official said.

"Ms. Darcy had the authority to make the decision on behalf of the Department of the Army, and she did so," Darcy spokesperson Moira Kelley told NBC News Tuesday evening.

Sunday's decision was hailed as a victory by protesters who oppose the pipeline, saying its construction threatens land believed to be sacred to the Standing Rock Sioux tribe and could threaten its drinking water. The activists call themselves "water protectors."

More from NBC News:

Cruel winter could be another blow to pipeline protesters' hopes

'Water is life': A look inside the Dakota Access Pipeline protesters' camp

Pakistan passenger plane carrying dozens goes missing en route to Islamabad

Darcy said in the memo that more work needs to be done in looking for an alternate route, with greater participation from the tribe.

The memo from Darcy makes clear that her decision "does not alter the Army's position that the Corps' prior reviews and actions have comported with legal requirements."

When asked why Assistant Secretary Darcy decided to go against the Corps of Engineers recommendation, Kelley her spokesperson said, "the Army decided that the totality of circumstances call for additional analysis, a more robust consideration of alternatives, and additional public information."

Army Secretary Eric Fanning was "supportive" of Darcy's decision but ultimately the decision was hers, the spokesperson said.

President Barack Obama appointed Darcy to be Assistant Secretary of the Army (Civil Works) in August of 2009.

The pipeline project spanned 1,172 miles and was projected to transport around 470,000 barrels of oil a day, with a maximum capacity of 570,000 barrels a day, according to the memo.

The portion that was to cross Lake Oahe was a half-mile upstream of the northern boundary of the Standing Rock Sioux reservation.

Darcy's memo says that documents which were part of an environmental assessment — which included a "Lake Oahe Crossing Spill Model Discussion" — were marked as confidential due to security concerns and were withheld from the public and the tribe.

Energy Transfer Partners, the company behind the project, has said that fears of impact on the tribe's water supplies were unfounded. The company on Sunday said the decision to deny the permit for more analysis was "a purely political action."



An Army Corps of Engineers official said that they "fully support the Army's decision," but confirmed that they did recommend that the pipeline construction be allowed.

The official said that the continued construction of the Dakota Access pipeline "met the requirements and legal standing" but that they "stand by the administration's decision."

Standing Rock Sioux Chairman Dave Archambault II dismissed any controversy over Darcy's decision going against the Corps' recommendations.

"As chairman, I get recommendations that come across my desk everyday. I don't always do what they say, I do what is best for my people," he said.

"We believe that she made the right decision because this is going to have an impact on our people, the pipeline would threaten our water, our culture, our heritage," Archambault II said. "This decision took a lot of courage and we commend her for it."

Despite the ruling in their favor over the weekend, many of the anti-pipeline activists fear the decision could be reversed in a Donald Trump administration.