Law enforcement officials fired at an unmanned aircraft and a large group of opponents of the Dakota Access pipeline blocked a North Dakota state highway Sunday, capping a weekend of protests.

A helicopter helping monitor a protest against the four-state pipeline Sunday morning was approached by a drone in a "threatening manner," the Morton County Sheriff's Office said in a statement. An officer in the helicopter told law enforcement on the ground that the pilot and passengers were "in fear of their lives" and that the unmanned aircraft was going after them. Less-than-lethal ammunition damaged the droned, which was then landed by its operator.

Morton County Sheriff Kyle Kirchmeier said drones flying near protests and near the area where hundreds have been camping out are not being operated according to federal regulations.

"Reports of drones not being operated within the (Federal Aviation Administration) guidelines or in a reckless and unsafe manner are being investigated and forwarded to the Morton County States Attorney's office for prosecution," Kirchmeier said in a statement. Two people operating drones during the protests have already been charged.

After 2 p.m. Sunday, the large group of protesters had blocked the highway with hay bales, rocks, tree stumps and other items to create a roadblock north of the encampments, to where at times thousands of people have flocked. The North Dakota Department of Transportation closed the highway down.

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Texas-based Energy Transfer Partners is building the $3.8 billion pipeline, which crosses through North Dakota, South Dakota, Iowa and Illinois. Opponents worry about potential effects on drinking water on the reservation and farther downstream because the pipeline will cross the Missouri River, as well as destruction of cultural artifacts.

Sunday's demonstrations come after more than 80 people were arrested Saturday during a large protest at a pipeline construction site. More than 220 people have now been arrested since demonstrations began in August.