A dog has been killed and several others injured after a snowmobile was deliberately driven into two dog-sled teams competing in a race, officials have said.
The incident happened during the Iditarod Trail Sled Dog Race in Alaska.
Race marshals said a man on a snowmobile first attacked one sled and later hit the sled that was following behind.
A man has now been arrested, but police say the motive remains unclear.
Arnold Demoski, 26, faces charges of assault, reckless endangerment, reckless driving and criminal mischief, Alaska State Troopers say.
Mr Demoski had earlier told the Alaska Dispatch News that the incident was not intentional, but that he had blacked out after drinking.
Race officials said veteran competitor Aliy Zirkle first reported being attacked near the village of Nulato on the Yukon River during a leg of the 1,000-mile (1,609km) race to Nome.
Alaska State Troopers spokeswoman Megan Peters told AP news agency that the snowmobile had hit the side of Ms Zirkle’s sled and the driver threatened her several more times before driving off.
One of her dogs was injured. She reported the incident after arriving in Nulato in the early hours of Saturday.
Some time later, competitor Jeff King, who had been following Ms Zirkle, reported being hit by the snowmobile in the same area. One of his dogs was killed and at least three others were injured.
“Regrettably, this incident very much alters the race of the two mushers competing for a win. However, both are going to continue on their way toward Nome,” the Iditarod Trail Committee said.
The Iditarod Trail Sled Dog Race is an annual event that takes place through some of Alaska’s most remote areas between Anchorage and Nome. Competitors and their teams often encounter blizzards, gale-force winds and freezing temperatures.