FARGO, N.D. -

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.

The Morton County Sheriff's Department is heading up the probe of the Sept. 3 incident, after which 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 says the BIA is representing Native American tribes.

A North Dakota state agency that regulates private investigation and security firms is also looking into the incident.