(CNN) -- August Reiger was supposed to reunite with his family at the top of a scenic mountain trail in Ecuador.
But when his father, mother and younger brother arrived Sunday at the meeting point overlooking the town of Baños, the 18-year-old was nowhere to be found.
Authorities in the South American country have been searching for the Oklahoma high school valedictorian for days.
Family members say it appears he vanished without a trace after they split up during a hike in the mountainous area 100 miles south of the capital, Quito.
"Nothing makes sense to me. ... Everyone's baffled, because it's not a dangerous place," his father, Chris Reiger, told CNN. "There's no rebels or something like that that who kidnap people. I can't come up with a scenario that could make sense."
Teams of firefighters, police, army troops and village volunteers have combed the area, Chris Reiger said, rappelling over steep mountain slopes and using search dogs in their hunt.
"They're still searching in the mountain, but not with that kind of force. They're confident he's not up there stuck somewhere," Chris Reiger said. "I could see. I was up there. I knew what they were doing, and I don't know, they didn't find him."
Lira de la Paz Villalva, governor of Ecuador's Tungurahua province, said local authorities don't have any leads about what could have caused the teen's disappearance.
A new search operation is planned for Friday.
August, an avid Spanish speaker with a deep interest in indigenous cultures, had been looking forward to the Ecuador trip. The family had been scheduled to leave for a three-day tour in remote jungle areas on Monday.
But instead, they've been traveling to local villages with police, posting pictures of their son and asking for help finding him.
It's out of the question that August would have wandered off or could be trying not to be found, the teen's father says.
"I feel that he's particularly mature for his age. He's not the kind of kid that does crazy stuff," Chris Reiger said.
August had just graduated from the Classen School of Advanced Studies in Oklahoma City, said Shannon Schmoyer, Reiger's father's cousin. The 18-year-old had asked his parents for a trip to a Spanish-speaking country as a graduation present, she said.
According to CNN affiliate KOKH, about 150 volunteers have joined the search, with helicopters assisting the effort. Local authorities have been cooperative but don't have many leads, Schmoyer said.
In Washington, the U.S. State Department said its embassy in Quito was in touch with the Reiger family and "is monitoring the situation closely."
"Protecting the well-being of U.S. citizens is one of the department's highest priorities, and we take all such reports seriously," it said.
The family arrived in Ecuador on June 11, Schmoyer said. She described the missing teen as "caring, sensitive, family-oriented, soft-spoken, compassionate, social, inquisitive and wicked smart."
August plays piano and guitar, was a state science fair winner and had a full scholarship to the University of Oklahoma, she said.
In Oklahoma City, word of his disappearance has has shaken those who know the teen.
"I'm not sure what to do. I feel very helpless," neighbor Tim Smith said. "Everyone here who knows him, it's all over Facebook. We're all e-mailing each other, trying to keep each other apprised. I'm knocking on whatever doors I can think of to knock on."
Smith, who used to drive August to school in a car pool, said he's been reaching out to local lawmakers to draw attention to the case.
"We really need people to let the governor's office and congressmen know," he said. "We need somebody to really step up and try to push this through, so that it gets some more exposure."
August's father said he hopes publicity about the case will help authorities locate his son.
"He's not gone yet in my mind," he said. "They're going to find him."
Journalist Andres Lopez and CNN's Marlena Baldacci, Jill Dougherty and Joe Sutton contributed to this report.