The Morton County Sheriff's Department is heading up the probe of the September 3 incident on private land, after which private security guards and protesters reported injuries.
Tribal officials say about 30 protesters were pepper-sprayed and some were bitten by dogs at the construction site near the Standing Rock Indian Reservation.
The task force includes members of the Morton and Mercer County sheriff's departments, the state Bureau of Criminal Investigation and the federal Bureau of Indian Affairs. Morton County Sheriff Kyle Kirchmeier said the BIA is representing Native American tribes.
"I am using all tools possible to insure this investigation is carried out with no bias toward Dakota Access pipeline nor the pipeline protesters," Kirchmeier said in a statement.
The sheriff's office would not name any of the security firms being investigated. The 1886 km Dakota Access pipeline is to begin in the western oil patch in North Dakota and run through South Dakota and Iowa before ending in Illinois.
Most recently, three federal agencies ordered a temporary halt to construction on US Army Corps of Engineers' land near a reservoir on the Missouri River in North Dakota and asked Energy Transfer Partners to stop work on a 40-mile span; ETP hasn't indicated whether it heeded the government's request that it "voluntarily pause" work.
A North Dakota state agency that regulates private investigation and security firms is also looking into the incident.
Monte Rogneby, an attorney representing North Dakota Private Investigation and Security Board, did not immediately return a message seeking comment.
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