The FBI's Child Abduction Rapid Deployment Team and other law-enforcement have entered the widening search for 16-year-old Selina Not Afraid.
Big Horn County Sheriff Pete Big Hair said authorities in Carbon and Yellowstone counties have also helped.
The sheriff said investigators are now looking at gas stations for information on an old green station wagon.
That car, possibly a Subaru with Wyoming plates, was seen at the rest area on Jan. 1.
The FBI team responds when a child under 18 is believed to be in danger.
Agents have set up at the county building in Hardin in an effort to locate Not Afraid, who has been missing since New Year's Day.
The Bureau of Indian Affairs has a command post at the rest area between Billings and Hardin, where Not Afraid was reportedly last seen.
Big Hair said there have been conflicting stories from the five people who rode with Not Afraid on New Year’s Day.