The Army Corps of Engineers dealt a blow to the progress of the controversial Dakota Access Pipeline on Monday, saying in a letter that more analysis and discussion with the Standing Rock Sioux tribe is needed before construction can take place under the Missouri River.

The company behind the construction of the Dakota Access Pipeline, Energy Transfer Partners, needs an easement, or permission, from the Corps in order to drill under Lake Oahe — on the Missouri River — to finish the oil pipeline along its proposed route.

Energy Transfer Partners had been waiting on a decision from the Army Corps since September when they launched a review of a requested easement.

Monday's letter from the Army Corps to the Standing Rock Sioux Tribe, Dakota Access LLC, and Energy Transfer Partners said construction "cannot occur because the Army has not made a final decision on whether to grant an easement." It said they would work with the Standing Rock Sioux on a timeline "that allows for robust discussion and analysis to be completed expeditiously."

The pipeline's proposed route has inspired protests from hundreds of Native American tribes, environmentalists and others calling themselves water protectors. Since August, they have descended upon Standing Rock, North Dakota near the pipeline's proposed site to stand in solidarity against the oil pipeline's construction.

Related: Dakota Access Pipeline: What's Behind the Protests?

Energy Transfer Partners CEO Kelcy Warren referred to the protesters as “violent mobs” in an interview with NBC News last week.

Warren at the time said the tribe's worries that the pipeline would destroy its sacred sites and compromise its water supply "were not based on the facts" and that the pipelines were prepared to withstand any rupture.

Energy Transfer Partners did not immediately respond to a request for comment on Monday.

The company said last week that the construction on either side of Lake Oahe was complete, and that they were "mobilizing horizontal drilling equipment to the drill box site in preparation for the tunneling under Lake Oahe."

But in the letter Monday, Army Corps Assistant Secretary of the Army Jo-Ellen Darcy asked for caution and for more tribal input, writing, "The Army is mindful of the history of the Great Sioux Nation's repeated dispossessions, including those to support water-resources projects. This history compels great caution and respect in considering the concerns that the Standing Rock Sioux Tribe has raised regarding the proposed crossing of Lake Oahe north of its reservation."

The "history" likely refers to the Army Corps' itself taking hundreds of thousands of acres of land from Native Americans when they built the Oahe Dam in the middle of the 20th Century.

The letter went on to recognize that portions of the lake fall within the Standing Rock Sioux Tribe's reservation boundaries and that the Tribe retains hunting and fishing rights in the lake.

Protesters demonstrate against the Energy Transfer Partners' Dakota Access oil pipeline near the Standing Rock Sioux reservation in Cannon Ball, North Dakota, U.S. September 9, 2016. ANDREW CULLEN / Reuters

Tribal members and their allies have maintained that the pipeline's proposed route also cuts across sacred land, including ancestral burial sites. An oil spill would also pose a risk to their drinking water, for which they and millions of others rely on the Missouri River.

Related: What Will a Trump Presidency Mean for the Dakota Access Pipeline?

In a statement, Standing Rock Sioux Chairman Dave Archambault II responded to the Army Corps letter, saying: "We are encouraged and know that the peaceful prayer and demonstration at Standing Rock have powerfully brought to light the unjust narrative suffered by tribal nations and Native Americans across the country." He pointed out that the 1,170-mile pipeline was rerouted from its original route near urban Bismarck after citizens there raised concerns about their own water safety. It was then moved closer to the Standing Rock Sioux reservation.

Chairman Archambault also called for continued peaceful and prayerful support of water protectors, writing, "The whole world is watching and where they see prayerful, peaceful resistance, they join us."