The Latest: Law enforcement ready for pipeline protesters

The Latest on the four-state, $3.8 billion Dakota Access oil pipeline (all times local):

1:30 p.m.

Law enforcement is on heightened alert for protest activity in the area where the developer of the Dakota Access oil pipeline is drilling under a Missouri River reservoir in North Dakota.

Energy Transfer Partners got the needed permission from the Army on Wednesday night to lay pipe under Lake Oahe. Work started right away on the last chunk of construction for the $3.8 billion pipeline to carry North Dakota oil to Illinois.

Opponents have camped in the area for months, often clashing with police. There have been nearly 700 arrests since August.

Highway Patrol Lt. Tom Iverson says there have been no incidents since drilling began, but law officers are ready.

ETP has its own security around the drill area, which isn't accessible by road from the protest camp.

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8:45 a.m.

One of two American Indian tribes fighting the Dakota Access oil pipeline has filed a legal challenge to try to block its completion.

The Cheyenne River Sioux worries a pipeline leak could contaminate its drinking water.

The tribe filed a legal challenge in federal court in Washington, D.C., on Thursday. The Army on Wednesday gave Energy Transfer Partners formal permission to lay pipe under a Missouri River reservoir in North Dakota. That's the last big chunk of construction for the $3.8 billion pipeline to carry North Dakota oil to Illinois. The work is underway.

The Dallas-based pipeline developer says it will be safe.

President Donald Trump signed an executive action in January instructing the Army Corps of Engineers to advance pipeline construction. The tribes argue that violates treaty rights.

The Standing Rock Sioux tribe has also vowed to fight the construction in court.

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7:55 a.m.

The company building the disputed Dakota Access oil pipeline has started construction on the final stretch of the $3.8 billion project.

Dallas-based Energy Transfer Partners began work late Wednesday after getting final permission from the Army to proceed with a crossing of the Missouri River in southern North Dakota. Company spokeswoman Vicki Granado says work commenced immediately after the company received permission.

The work had been stalled for months due to opposition by the Standing Rock Sioux, but President Donald Trump last month instructed the Army Corps of Engineers to advance pipeline construction.

The tribe fears a pipeline leak could contaminate its drinking water. ETP says the pipeline is safe.

Granado says it will take about two months to complete drilling under the lake.

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1:15 a.m.

The company building the Dakota Access oil pipeline says it plans to immediately resume construction on the long-stalled project.

Dallas-based Energy Transfer Partners on Wednesday got final permission from the Army to lay pipe under a Missouri River reservoir in North Dakota. The project had been delayed for months before President Donald Trump last month instructed the Army Corps of Engineers to advance work on the pipeline.

The Standing Rock Sioux tribe has vowed to continue fighting the construction. Tribal Chairman Dave Archambault said in a statement Wednesday that the Standing Rock Sioux are prepared to continue battling the pipeline "in the courts."

The tribe fears a pipeline leak could contaminate its drinking water. But ETP contends the pipeline is safe.

Opponents of the project held demonstrations Wednesday in several cities, including Chicago, New York and Washington, D.C.