(CNN) -- Japanese authorities will continue searching for a missing U.S. university professor on Wednesday, but they will reduce the size of search team, a police official said Tuesday.
Craig Arnold was doing research for a poetry and essay book on volcanoes, a colleague said.
Craig Arnold, an award-winning poet and assistant professor at the University of Wyoming, disappeared April 27 after starting a hike to Shintake volcano on the island of Kuchinoerabujima, the school said.
Requesting anonymity, the official from Yakushima Police Station said the number of searchers will come down from 18 to three on Wednesday. The search team, unprecedented in scale for this tiny island, has so far found no trace of the missing professor, said the official.
Arnold's family has hired a private, U.S.-based rescue group that is set to join the search in Japan this week, according to his sister-in-law, Augusta Palmer.
Searchers found footprints on a trail leading to the volcano but have "conclusively ruled" that he didn't fall in at the top, said Palmer, of Brooklyn, New York.
Authorities speculate that Arnold, 41, reached the top but ran into some type of problem coming down, possibly going off a trail into heavy vegetation, Palmer said Monday. She got her information from her husband, Craig's brother Chris, who arrived on the island on Sunday to monitor the search.
"If you're off the trail, there are a lot of holes in the ground that you might not see coming, especially if it's getting dark. So [searchers] are going to the places where they know there are the worst holes, and they've been lowering people on ropes to check," Palmer said.
A police spokesman in Japan said last week that although search efforts normally end after three days, the search for Arnold was extended after a friend of the professor claimed the educator's Facebook account was accessed for about a minute on Thursday.
The University of Wyoming has said that state's congressional delegation has been in touch with U.S. and Japanese officials to request extensions.
Investigators don't know the significance of Arnold's Facebook page being accessed last week, Palmer said. He was carrying a cell phone, but it appears the phone couldn't get reception on the island, she said.
Arnold, a creative-writing professor, was doing research for a poetry and essay book he is writing about volcanoes, according to Peter Parolin, head of the university's English Department. Arnold is an experienced volcano hiker, and he took a semester off from teaching and traveled to Japan alone through a U.S.-Japan creative artists' fellowship, Parolin said.
A fund supporting efforts to find Arnold, linked in a "Find Craig Arnold" Facebook page, has collected more than $9,000, Palmer said. The money is helping to pay a private U.S.-based search and rescue group to send a team of four to the island, Palmer said.
The team was to arrive on the island Tuesday, Palmer said.
Four U.S. Air Force helicopters from Kadena Air Base on Okinawa participated in the search late last week, the U.S. Embassy in Tokyo said.
"The entire family is so grateful for all the people who have helped us," Palmer said. "That extends to the Japanese authorities and people who contributed to the fund."
Japanese police said few locals attempt to hike Shintake volcano. The island has about 160 residents.
Arnold, according to the University of Wyoming, has written 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.
CNN's Yoko Wakatsuki contributed to this report