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Posted: 10:55 AM- A Minnesota professor missing in the Needles District of Canyonlands National Park left suicide notes for relatives and friends before he disappeared while backpacking last week.

Police in Minnesota say Jerry O. Wolff, a professor of biological sciences from Sartell, Minn., sent letters about his plans to commit suicide and "return my body and soul to nature," The Star Tribune newspaper in Minneapolis reported this morning.

Park rangers from Canyonlands and nearby Arches National Park began a manhunt May 15 for the 65-year-old St. Cloud State University professor but so far haven't found him.

"We've found just no evidence of him at all," Canyonlands spokesman Paul Henderson said.

Wolff wrote in one of the letters to relatives that he was troubled over a relationship, according to The Star Tribune. The newspaper reported the man had recently moved out of the home of his partner Shawn Thomas and her two school-age daughters.

Rangers were forced to scale back the search Monday because of hot weather and obligations to arriving Memorial Day weekend visitors.

Wolff was last seen headed to the remote and rugged area near Horse Canyon and Salt Creek Canyon to camp, Henderson said. He flew into Moab on May 10 and hired a local shuttle company to drop him at the Needles District ranger station. There, he picked up his camping permit, good for five nights, and "took off," Henderson said.

Wolff didn't make a reservation with the shuttle company for a return pick up, Henderson said.

Wolff did not return to the ranger station on May 15 when he was scheduled to, prompting family and friends to report him missing.

Thomas told the The Star Tribune that she's received one of the suicide notes from Wolff. She called the man an "extremely stressed person" but said he didn't have overwhelming problems that he couldn't work through, according to the newspaper.

Search crews were mobilized the same day and began an extensive grid search for Wolff. More than 20 people, as well as two search dog units and aerial support looked for him.

"We threw as many resources at it as we could," Henderson said.

Crews searched "all logical places," but could not locate his campsite or equipment.

Wolff, who received degrees in the 1970s from the University of Alaska-Fairbanks, and Northern Arizona University in Flagstaff, was not conducting research in the area, Henderson said. He was apparently prepared for a recreational trip and had made his reservation to camp in the area in early April.

Needles District rangers will continue looking for Wolff and dog teams and planes will be brought in to help at least once a week, Henderson said.

Fliers with Wolff's likeness and description will be posted in the Canyonlands area this weekend so visitors can keep an eye out for him, Henderson said.