(CNN) -- Japanese police and U.S. Air Force helicopters were searching Friday for an American university professor who disappeared on a tiny volcanic island in Japan.
Craig Arnold was doing research for a poetry and essay book on volcanoes, a colleague said.
Craig Arnold, a 41-year-old assistant professor at the University of Wyoming, did not return from his Monday hike to a volcano on Kuchinoerabujima, a small island in southern Japan, the school said.
"The only clues that [searchers] have found were indications that he had begun the ascent -- footprints on the trail," said Peter Parolin, head of the university's English department, citing Arnold's family.
Kazuko Watanabe, the owner of the inn where Arnold checked in Monday, said that Arnold had a cup of tea before he left for the hike but did not seem to take food or water with him.
Local police said 40 people on the island of only 160 residents were searching for Arnold on Friday. Four U.S. Air Force helicopters from Kadena Air Base on Okinawa were also diverted to aid in the search, the U.S. Embassy in Tokyo said.
A police spokesman said search efforts normally end after three days, but the search for Arnold continued after a friend of the professor claimed the educator's Facebook account was accessed for about a minute on Thursday.
Arnold, a creative writing professor, was doing research for a poetry and essay book he is writing about volcanoes, Parolin said. See where island is located »
Arnold, according to the school, wrote two award-winning volumes of poetry: "Shells," chosen for the Yale Series of Younger Poets in 1999; and 2008's "Made Flesh."
His work has been anthologized in several volumes of the Best American Poetry Series, and his awards and honors include a Fulbright Fellowship and the Joseph Brodsky Rome Prize from the American Academy of Arts and Letters, according to the university.
Arnold took the semester off from teaching and traveled to Japan alone through a U.S.-Japan creative artists' fellowship, Parolin said.
He had planned to spend a day alone at the volcano, and the innkeeper contacted authorities when he didn't return that evening, according to Parolin.
Arnold has scaled many volcanoes, Parolin said.
"If it's technical or dangerous, he does it with a guide. But from all reports, on this trip he went by himself," Parolin said.
Japanese police said few locals attempt to hike the Shintake volcano.
Arnold "is the kind of person and poet who is attracted to extreme places and extreme geographies -- places that not all of us visit," the University of Wyoming English department head said.
"He feels the need to go to places that people don't go and come back and tell us about them," Parolin said.
CNN's Jason Hanna and Yoko Wakatsuki contributed to this report.
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