BISMARCK, N.D. (AP) — The Army on Friday formally ended further environmental study of the Dakota Access oil pipeline's disputed crossing beneath a Missouri River reservoir in southern North Dakota.

Meanwhile, its Corps of Engineers branch continued efforts to accelerate cleanup at a protest camp near the drilling site that's threatened by spring flooding.

The Corps launched the study on Jan. 18 in light of concerns from the Standing Rock Sioux and other Native American tribes that a pipeline leak beneath Lake Oahe would pollute drinking water.

President Donald Trump a week later pushed to advance pipeline construction, and the Army gave Texas-based developer Energy Transfer Partners permission for the crossing on Feb. 8. Work quickly began on the final chunk of construction.

Pipeline opponents have continued to call for more study despite the fact that ETP has said the $3.8 billion pipeline to move North Dakota oil to a shipping point in Illinois could be operating as early as next month. More than 100,000 comments had already been submitted for the study, according to the Indigenous Environmental Network.

The Army published notice Friday in the Federal Register that it was scrapping the study.

The Standing Rock and Cheyenne River Sioux also are fighting the pipeline work in court, with the next hearing set for Feb. 28. In the meantime, hundreds of pipeline opponents have continued to occupy a camp near the drilling site in North Dakota.

State and federal authorities have told the few hundred people remaining in the camp to leave by Wednesday. Authorities want the area cleaned and closed before spring floodwaters wash tons of trash and debris into nearby rivers, including the Missouri River, and cause an environmental disaster.

The tribe launched a cleanup effort in late January. The state and Corps were continuing Friday to try to line up additional contractors to speed up the work, according to Corps Capt. Ryan Hignight and Mike Nowatzki, spokesman for Gov. Doug Burgum.

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"We're running out of time," Hignight said. "We need to ensure that the land is remediated as soon as possible."

Some in camp think the flood fears are overblown and that authorities are trying to turn public sentiment against them.

"We're all working hard to get the lower (flood-prone) grounds clear," said Giovanni Sanchez, a Pennsylvania man who has been at the camp since November. "I think they're just trying to find any reason to get us out of here."

The latest spring flood outlook from the National Weather Service, issued Thursday, calls for minor flooding in the area. The outlook doesn't include flood risks associated with river ice jams, which can't be predicted.

The camp area has been the site of numerous and sometimes violent clashes between police and protesters. There have been more than 700 arrests in the region since August.

The head of the FBI in Minneapolis, which oversees the Dakotas, recently said his agents are working with local, state and federal agencies to investigate some activity at the protests near the site.

"We have not been standing out there at roadblocks, but ... there have been things that we would consider acts of terrorism — arson and so forth — that have taken place," FBI Special Agent in Charge Rick Thornton said.

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Associated Press writer Amy Forliti contributed to this story from Minneapolis. Follow her on Twitter at: http://www.twitter.com/amyforliti .