CANNON BALL, N.D. (AP) — Authorities say the use of water spray in below-freezing temperatures against Dakota Access oil pipeline protesters was necessary, and they won’t rule out doing it again.

Authorities used the tactic during a skirmish with protesters overnight in southern North Dakota. Morton County Sheriff Kyle Kirchmeier says authorities used fire hoses to repel aggressive protesters who were throwing objects including rocks, asphalt and water bottles at officers. One officer was hit in the head.

The human rights organization Amnesty International and other groups have denounced officers’ use of water when temperatures were in the 20s. Dallas Goldtooth with the Indigenous Environmental Network says at least 17 protesters were taken to hospitals, some with hypothermia.

Mandan Police Chief Jason Ziegler says authorities won’t rule out using water again if it’s deemed necessary.