Tensions continued in North Dakota on Monday afternoon as law enforcement officials arrested 16 people at a demonstration, one day after hundreds clashed with the police over the Dakota Access Pipeline.

During a news conference on Monday, officials also defended their use of fire hoses against protesters the night before, despite the below-freezing weather.

“Some of the water was used to repel some of the protest activities that were occurring, and it was used at a time where they were aggressive towards the officers,” the Morton County sheriff, Kyle Kirchmeier, said at the news conference.

In a statement late on Sunday, the Morton County Sheriff’s Department characterized the demonstration as an “ongoing riot,” releasing photos that it said showed protesters “setting fires and using aggressive tactics” while trying to dismantle a police barricade on Backwater Bridge, which has for months been the site of a protest against the pipeline.