CHARLESTON, W.Va. - The sister of a missing autistic hiker said Wednesday the family is worried that her brother won't be able to respond if he hears searchers calling his name. "That is our biggest concern, that someone calls for him and he cannot say, 'I'm here,' you know, 'Help me,'" Jacob Allen's sister, Brittany Allen, told CNN. She said her family normally uses pictures to communicate with her brother. But an official said Allen's parents were hopeful Wednesday, as the search for their son stretched into a fourth day. "The attitude here today is that today is going to be the day we find him," said Chris Stadelman, a spokesman for the search effort. "As time goes on they get more concerned but they're still very optimistic." Allen, 18, has been missing since Sunday afternoon, when he wandered away from his parents, Jim and Karen Allen of Morgantown, while hiking. Between 15 and 20 crews searched overnight. On Wednesday morning, busloads of volunteers gathered to continue the search. The volunteers will comb the same 10-square-mile area, because searchers believe their dogs may have picked up a scent overnight, "but nothing is firm on that," Stadelman said. Search crews have been concerned about temperatures, which were in the low 40s Wednesday morning and have dipped as low as 38 degrees. Hypothermia should not be an issue provided Allen stays dry, Stadelman said. It has not rained, but there was a chance of showers over the next couple of days. Allen was wearing hiking boots, a long-sleeved T-shirt, a wind jacket and wind pants. His hat was found Monday, near the area where he was last seen. He had no food or water with him, but Stadelman said there are natural water sources in the search area. There is no way of telling how Allen who has the mental capacity of a 3- or 4-year-old might react in this situation, said Stadelman. "Obviously this is something none of us has ever been through and certainly Jacob hasn't either," he said. Jim and Karen Allen went out several times with rescue crews but stayed only for brief periods so they could be ready to go to their son when needed, said Stadelman. "We don't want them to be far away when we find him," he said. "We want to get them to him as quickly as possible." The couple brought their son's pillowcase for rescue dogs to sniff, hoping the animals could pick up his scent. Helicopters are also being used in the search. While Allen is described as severely autistic by his mother, he is in good physical shape and likes to hike. http://news.yahoo.com/s/ap/20071017/ap_on_re_us/missing_hiker;_ylt=AuJqDnk8NcVL2s4.H4QWoSGs0NUE I prey they find him soon.