NEAR CANNON BALL, N.D.-The Standing Rock Sioux Tribe says it surveyed a portion of the Dakota Access Pipeline route this week and discovered multiple graves and other significant historical sites not previously identified.

Court documents filed Friday include statements from Tim Mentz, former tribal historic preservation officer, who surveyed about 2 miles of the pipeline corridor this week after receiving an invitation from the landowner.

Mentz writes in court documents that his survey located at least 27 burials, 16 stone rings, 19 effigies and other features in or adjacent to the pipeline corridor just north of the Standing Rock reservation.

"This concentration of stone features is very unusual and reveals that this was a culturally very important place for the tribe's ancestors," wrote Mentz, whose family owns a business that does archaeological and tribal identification survey work.

The Standing Rock Sioux Tribe is suing the U.S. Army Corps of Engineers for issuing a permit for the pipeline to cross the Missouri River less than a mile north of the reservation.

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The discoveries this week highlight deficiencies in the cultural resource inventory of the 1,172-mile pipeline route, said Jan Hasselman, an attorney for the environmental law firm Earthjustice that represents the tribe.

"This is exactly why the tribe was demanding from the very beginning to participate in surveys," Hasselman told Forum News Service late Friday. "The whole area is rich in cultural heritage, but the tribe never had the chance to go out and do this work."

The Corps of Engineers and Dakota Access had not responded to the court documents filed Friday.

In earlier filings, the corps argued that the agency had hundreds of consultations with federally recognized tribes, including Standing Rock. The corps said in court records that the agency attempted to meet with Standing Rock on other occasions, but the tribe declined to participate. In addition, the corps also required Dakota Access to offer tribal monitoring for certain areas during construction.

A federal judge heard arguments in the case on Aug. 24 and indicated he planned to rule by Sept. 9 on the tribe's request for an injunction to stop work on the pipeline.

Mentz wrote that some of the features he identified are "unquestionably" eligible for listing under the National Historic Preservation Act and he notes that one may be one of the "most significant archeological finds in North Dakota in many years."

Mentz surveyed an area about 1¾ miles from the Missouri River crossing that was subject to the Corps of Engineers permit.

Dakota Access hired archaeological firms to conduct a cultural resource survey of the route. The State Historical Society of North Dakota concurred with the findings that no significant sites would be affected by the pipeline construction.

But Mentz wrote that he has reviewed their survey and it did not include the sites he has identified. Two North Dakota historians also have raised questions about omissions in the cultural resource survey.

"The pipeline's hired guns don't get to decide what is sacred to the tribe," Hasselman said.

Mentz wrote in court documents that he received an unsolicited call on Aug. 28 from the man who owns the land north of the Standing Rock reservation inviting him to survey the area. Mentz concluded the survey work on Thursday and the tribe's attorneys filed the documentation on Friday.

Hundreds of pipeline opponents continue camping near the pipeline construction site north of Cannon Ball in protest of the river crossing. Pipeline construction has been suspended in that area, but continues elsewhere. The $3.8 billion pipeline, expected to be in service by the end of the year, would transport crude oil from North Dakota to Patoka, Ill.