Protesters broke through a fence line at an alternate construction site for the Dakota Access Pipeline early Saturday afternoon south of Mandan.
Three private security officers at the site were injured by protesters, said Donnell Preskey, spokeswoman for the Morton County Sheriff's Office. One of them required hospitalization. Two security K-9s were also taken to veterinarians to be treated for injuries.
"Any suggestion that today's event was a peaceful protest is false. This was more like a riot than a protest," said Morton County Sheriff Kyle Kirchmeier. "Individuals crossed onto private property and accosted private security officers with wooden posts and flag poles."
"The aggression and violence displayed here today is unlawful and should not be repeated. While no arrests were made at the scene, we are actively investigating the incident and individuals who organized and participated in this unlawful event," he said.
Shortly before 9 p.m., Tribal Chairman Dave Archambault II issued a statement saying sacred places containing ancient burial sites, places of prayer and other significant cultural artifacts of the Standing Rock Sioux Tribe were destroyed Saturday by Energy Transfer Partners.
"This demolition is devastating," Archambault said. "These grounds are the resting place of our ancestors. The ancient cairns and stone prayer rings cannot be replaced. In one day, our sacred land has been turned into hollow ground."
According to the news release from Archambault, construction crews removed topsoil across an area about 150 feet wide, stretching for two miles, northwest of the confluence of the Cannonball and Missouri rivers.
At about 2:30 p.m., protesters crossed through an area on the west side of Highway 1806, near the Missouri River and Cannon Ball, said Preskey. According to several reports from security officers, knives were pulled on them or they witnessed protesters with large knives.
Protesters reportedly marched from their camp located on U.S. Army Corps of Engineers land and marched to the construction site, said Preskey on the west side of Highway 1806. Once the marchers arrived at the construction site, they broke down a wire fence by stepping and jumping on it, the marchers estimated to be at least 300 people, broke into the construction area and rushed construction workers, she said.
By late Saturday afternoon, the area had been cleared. No arrests were made, Preskey said.
Witnesses also say they saw protesters climb onto vehicles at the construction site and beat on them, trying to break the windows.
Two phone messages were left for Johnelle Leingang, emergency manager for Standing Rock, and LaDonna Brave Bull Allard, co-organizer of the protest and tribal historian, but they could not immediately be reached for comment.
Tim Mentz, former tribal historic preservation officer, said in a statement he surveyed the land and confirmed there were multiple graves and prayer sites where the construction site was breached Saturday.
One security guard said a crowd of protesters pushed him against a vehicle and said he was jabbed in the side with a wooden fence pole and kicked in the knees. This caused him to fall to the ground. He heard people saying, "stomp him, kick him, he's just not to going to leave," said Preskey. That security guard was transported to a Bismarck hospital.
Investigators have taken three formal reports of injuries from security officers. However, witnesses say several additional private security officers were assaulted.
Law enforcement from Morton County, Burleigh County and Highway Patrol responded to the construction site. The Morton County Sheriff's Department in cooperation with the Bureau of Criminal Investigation is investigating the incident.