The staff at Rocky Mountain National Park continue to search for a hiker who was last seen near the summit of Fairchild Mountain Sunday afternoon.Hillel Ben-Avi, 45, was hiking with his brother in the Mummy Range in the northern part of the park when they became separated.Ben-Avi was hiking ahead of his brother and had reached summit of Fairchild Mountain at approximately 1:30 p.m. Sunday. However, when his brother scaled the peak, Ben-Avi was not there.Park dispatch received a cell phone call from Ben-Avi at approximately 4:30 p.m. and then again almost 30 minutes later. He said that he was lost and had mentioned that he had hiked by three lakes.A park ranger told him to return to the last lake he had passed, but then the cell phone signal was lost.Another call was made from his cell phone about five hours later but no connection was made, rangers said.The search for Ben-Avi began Sunday evening and continued into Monday, with rangers and other park staff searching near lakes, trails and drainages in the Mummy Range.A helicopter also helped with the search efforts Monday, but there was no sign of Ben-Avi.Another day of intensive searching began Tuesday morning with approximately 70 people, two dog teams and two helicopters, including one with thermal imaging capabilities, involved.Larimer County Search and Rescue, Rocky Mountain Rescue Group and Alpine Rescue are all involved in the search.Ben-Avi has brown hair and brown eyes and was last seen wearing a blue jacket with orange sleeves, gray pants, green and gray baseball cap and carrying a blue-green Jansport back pack.Although he is described as athletic, he was not prepared to spend the night in the backcountry, rescuers said. Ben-Avi is visiting from Austin, Texas.Officials are asking park visitors who might have been hiking in the Mummy Range area on Sunday or Monday to call park officials at (970) 586-1204. They need help determining where Ben-Avi might have been seen last.The Mummy Range is a remote, high altitude, rugged area which includes Fairchild Mountain, Mount Chapin, Mount Chiquita, Ypsilon Mountain and other prominent peaks. There are a few designated trails in the area but hiking mainly occurs in the backcountry.