"We just don't know," Cannon said. "The scene wouldn't look any different."
Haun had been at a party at a Payson home hours earlier with other teenagers who had been drinking, Cannon said. At some point, Haun left and may have been walking back home to Spanish Fork, according to the sergeant.
Initially, other teens at the party were reluctant to talk to police, but Cannon said that in the last two months investigators have made contact with nearly everyone who attended the party, and no leads have solidified.
"A number of them were afraid initially," Cannon said. "Most of the people who were there were underage and had been drinking. ... Frankly, when you compare it with what we really want to find out, we have priorities that are higher. Finding out what happened to Nathan is a higher priority than giving a ticket for illegal consumption."
Haun's body was found by a woman around 5:30 a.m. near 2000 Arrowhead Trail, a rural road used by locals as a shortcut between Spanish Fork and Payson.
Cannon said Thursday that investigators are looking into a piece of a vehicle found in the area that had "evidence from Nathan" on it. They are trying to determine if that piece was part of the vehicle that hit Haun, or if it was already lying near the roadway before the accident.
He said they also have investigated leads gleaned from interviews, but so far, they are no closer to solving the case.
"There's a number of possibilities," Cannon said. "We just don't have anything right now pointing one way or another."
A $5,000 reward has been offered by the sheriff's office to anyone with information that can help them solve the mystery.
Detectives are asking anyone with information that might help them find the vehicle and the driver to all the Utah County Sheriff's Office at 801-794-3970.