It's been almost a month since Tennessee teenager Joe Keller, who was on a cross-country trip with his friends, went for a run and disappeared in a case that has left investigators baffled.

Keller, 19, disappeared 10 days into a 15-day road trip from Tennessee to the West Coast. He was travelling along with two of his friends, Collin Gwaltney and Christian Fetzner, according to the Daily Mail.

Keller and Gwaltney went for an afternoon run in Antonito, Colo. on July 23, but since Keller couldn't catch up to his friend, the two decided to regroup at a ranch.

That was the last time Keller was seen, and local investigators, as well as detectives from Keller's home state, have found no trace or evidence of him after weeks of searching with helicopters, horses, dogs and drones, Fox News reported.

Keller's family, desperate for answers, has offered a reward of $50,000 for information that might help them find him.

"It's just an awful situation not knowing if your loved one is in heaven or in a terrible situation on earth or out in the wilderness," said Keller's mother, Zoe, according to NBC News.

"Anything is possible at this point," said Keller's father, Neal, who is in Colorado searching for his missing son. "There has been no sign, no clue, no nothing."

Neal believes more resources are required to find his son. "The sheriff's department out here has limited resources and have done what they can, but we really need more help," he said.

A Facebook page called "Find Joe Keller" has also been set up by Keller's friends. The page describes him as 5'7" tall, weighing 160 lbs with blond hair and hazel eyes. He was last seen wearing red shorts with reflective strips on the sides and blue running shoes.