– A joint task force of North Dakota and federal officials is investigating a clash between Dakota Access pipeline protesters and private security guards earlier this month, a county sheriff announced Tuesday.

The Morton County Sheriff’s Department is heading up the probe of the Sept. 3 incident on private land, after which private security guards and protesters reported injuries.

Tribal officials said 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.

Kirchmeier said the investigation will determine which firms were hired to provide security that day and whether they were licensed. The task force also is looking into whether tribal artifacts were disturbed at the site as the Standing Rock Sioux tribe has argued.

“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 1,172-mile 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. The pipeline is being built by Dallas-based Energy Transfer Partners, which said it intends to finish the project despite numerous legal delays. Most recently, three federal agencies ordered a temporary halt to construction on U.S. 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 the North Dakota Private Investigation and Security Board, did not immediately return a message seeking comment.

Standing Rock Tribal Chairman David Archambault II told members of the U.N. Human Rights Commission in Geneva on Tuesday that human rights have been violated by the construction of the pipeline and the actions of law enforcement.