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The search is still on for a man who fell into a hot spring at Yellowstone National Park.

Witnesses said the man, described as being in his early 20s, walked off the boardwalk and into the hot spring in the Norris Geyser Basin on Tuesday afternoon. Officials didn’t say whether the man was a tourist.

“At this time, rangers are treating this incident as a probable fatality because the victim has not been located,” Jody Lyle, a spokeswoman for the park, said several hours later.

Authorities closed the basin and were proceeding with caution because of the heat around the springs.

The park website describes the site as the hottest thermal area in Yellowstone.

Over the weekend, a 13-year-old boy burned his ankle and foot when he fell into a hot pool in Yellowstone’s Upper Geyser Basin. His father slipped while carrying him, the Billings Gazette reported.