A heavily armed man was arrested on Thursday as he was caught entering a camp for Native American water protectors protesting near the Standing Rock Indian Reservation in North Dakota, local media reported.
The unidentified suspect is an alleged security agent employed by the company behind the controversial Dakota Access Pipeline.
Protesters reported a man driving down Highway 1806 in a white Chevy Silverado with an assault rifle in the cab. The road is the main site of a conflict between local tribes, who want to halt construction of the US$3.8 billion Dakota Access Pipeline.
The event takes place as protests against the project have escalated with violent crackdowns and arrests by police.
More than 100 heavily-armed police officers from several regional police agencies were deployed this week to disperse the protest, on Wednesday they used water cannons, pepper spray, and concussion grenades on protesters, arresting 141 protesters and injuring dozens.
Riot police shot beanbag rounds at some of the estimated 330 protesters as helicopters flew overhead. Protesters – mostly Native American – have been demonstrating for several months.
The protest has grown into a larger movement in the United States, drawing in other tribes and environmentalists, it has also been an issue in the agenda of the presidential candidates Hillary Clinton and Donald Trump.