BISMARCK, N.D. (AP) – American Indians blocked crews constructing the $3.8 billion Dakota Access oil pipeline in North Dakota on Wednesday, while Iowa landowners are asking a judge to halt work in their state until their constitutional challenges are heard.

Law enforcement was called to keep the peace between protesters and armed security guards hired by the Texas-based developer of the four-state pipeline as crews worked on the part of the pipeline that’s just north of the Standing Rock Sioux reservation in southern North Dakota. There, at the confluence of the Cannonball and Missouri rivers, American Indians have for months been staging a nonviolent protest at a “spirit camp.”

“Everybody is nonviolent and peaceful,” said LaDonna Brave Bull Allard, a tribal historian at Standing Rock. “We want to hold them back until we can get to court.”

A dozen Iowa landowners are hoping for the same thing, asking a state court in documents filed Tuesday in Des Moines to immediately hear their motion and keep the company from digging a trench across their land. Attorney Bill Hanigan says the Iowa Utilities Board misinterpreted a 2006 state law that bans agricultural land from being taken for private projects via eminent domain.

The 1,172-mile pipeline planned by Dallas-based Energy Transfer Partners starts in North Dakota and passes through South Dakota and Iowa before ending in western Illinois.

Energy Transfer Partners officials didn’t return phone calls Wednesday seeking comment from The Associated Press, but said in an email that it is continuing construction in Iowa on schedule. Work already has begun in North Dakota, South Dakota and Illinois.

The Standing Rock Sioux Tribe late last month sued federal regulators for approving the pipeline, which would be the biggest-capacity one to carry oil out of the western part of the state. The tribe argues that the pipeline would impact drinking water and sacred sites on its 2.3-million acre reservation straddling the North Dakota-South Dakota border.

About 45 construction vehicles were backed up along a road Wednesday afternoon, Brave Bull Allard said.

Highway Patrol Lt. Tom Iverson said five troopers were on scene Wednesday, along with deputies with the Morton County Sheriff’s office. No arrests had been reported, but crews had been blocked earlier in the day.

“We are there to ensure the safety of the motoring public, as well as all those involved,” Iverson said.

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Pitt reported from Des Moines, Iowa.