Aubrey Sacco's father, Paul Sacco, has a lot of confidence in his 23-year-old daughter. She's traveled to Japan alone and has volunteered in schools in India. Her trek to Langtang in Nepal, a 10-15 day hike, was just the latest excursion. But, now Paul Sacco, a Greeley attorney, is worried. His daughter was due back April 29, but he hasn't heard from her yet. She checked in at a national park on April 20, but did not check out. “What's really, really difficult for a parent is she could be on a detour by her own choosing or by necessity, or she could be in trouble,” Paul Sacco tells the Greeley Tribune , “and it's a terrible vacillation between those two possibilities.” A massive search is underway for Aubrey Sacco, a 2009 University of Colorado graduate in art and psychology, according to Boulder's Daily Camera . Paul Sacco is hopeful any Coloradans who hike in the region might have seen his daughter or may know someone traveling there who can aid in the search. Perhaps she has been unable to communicate, he says. "She's a courageous little girl," he tells The Denver Post . "She's fearless and she's very strong. My gut tells me she's fine, but we haven't heard from her and we're very worried." Anyone with information can contact the Sacco family via e-mail at email@example.com . The family has also created a Facebook page for Aubrey .