Police are Charles Langstonlooking for a 17-year-old foreign exchange student who went missing Thursday from the home he was sharing with his host family in Utah, police say.

The teen was last seen at about 3:30 a.m. local time in Riverdale, about 30 miles north of Salt Lake City, according to the Utah Department of Public Safety.

Kai Zhuang of China was officially declared missing Friday, and police have “reason to believe” that Zhuang is “being held against his will," police have told media outlets. The department did not immediately respond to request for comment from USA TODAY on Friday.

Kai is described as 5 feet, 6 inches tall, 150 pounds with black hair and dark eyes. It's unclear what he was wearing when he was last seen, police say he often wears a green jacket.

YouTuber helps locate missing person:Human remains found in Missouri pond

Riverdale police chief says teen's parents received ransom note

Police decided to send out an endangered missing person advisory early Friday after the department received a call from Kai's high school saying they'd heard from the teen's parents in China.

The couple claimed to have received a photo of their son along with a ransom letter, Riverdale Police Chief Casey Warren said in a press conference held Friday.

Officers contacted the host family who were unaware the teen was missing, he said.

Warren said there is no evidence to suggest that Kai was taken from his home by force, contradicting a previous statement from police that “officers have reason to believe (Kai) has been forcefully taken from his home and is being held against his will.”

An Amber Alert for Kai wasn't issued because there is no suspect yet in the case, Warren said.

Riverdale police are working with the FBI, the U.S. Embassy in China and Chinese officials to bring the missing teen home.

"At this point, the information we have is very limited," Warren said.

Anyone with information is being asked to call Riverdale police at 801-394-6616 or Weber County dispatch at 801-395-8221.