Kasey Vang, 15, was reported missing from Bellevue, Wisconsin, Jan. 11, according to the Brown County Sheriff's Office.
Vang was last seen Saturday with her younger sister in Milwaukee, more than 100 miles from her hometown. Vang's sister has since returned home and is speaking with investigators.
"Kasey is believed to be a victim of human trafficking by an unknown adult male," the sheriff's office said in a news release Monday.
Lt. Jim Valley of the Brown County Sheriff's Office said the girls "weren't necessarily kidnapped or taken."
"We believe that they possibly knew this individual through some means and went with him down to that area," Valley told ABC Green Bay affiliate WBAY-TV.
Investigators believe Vang and her sister left together and were separated at some point. Both are believed to be victims of human trafficking.
Investigators are working to determine the identity of the man.
Anyone with information on Vang's whereabouts is asked to contact the Brown County Sheriff’s Office at (920) 448-6192.