Two inmates are dead and several others are injured after a “group disturbance” at a privately run prison in southern Oklahoma, according to prison authorities.

The incident took place at Lawton Correctional and Rehabilitation Facility on Friday and is being investigated by the Oklahoma Department of Corrections, according to a statement from Kay Thompson, the department’s chief of public relations.

Thompson said in an email to CNN on Saturday the incident took place after an “operational error” allowed “the disturbance to happen.” She said the disturbance was related to “security threat groups,” which Thompson said was another term for gangs.

As a result, the facility is locked down and all visitations are canceled through the weekend.

A spokesperson for The GEO Group, a security company that operates prisons and detention facilities under contract with governments, told CNN the deaths and injuries resulted from “an inmate-on-inmate assault.”

Thompson did not name the two patients who died in the statement. In addition to the two deaths, two inmates were seriously injured. One was taken to the hospital by med flight and the other by vehicle. Many others were treated at the facility with minor injuries, but the exact number is unknown, according to Thompson. One officer also sustained minor injuries, according to The GEO Group.