The federal safety agency said in documents posted on its website that it has received 12 complaints of fires. Five of the blazes destroyed the cars. Eight fires happened when the engines had been shut off.
No injuries have been reported, NHTSA said in documents posted during the weekend.
The agency says there's an increasing trend of fires, with most of the complaints coming in the past year. NHTSA opened a preliminary evaluation on Oct. 11 and will try to find the cause of the blazes. Depending on the findings, a recall could be ordered.
The fires could affect "S" models of the Mini Cooper, Cooper Clubman and Cooper Convertible, the agency said.
A spokeswoman for Mini, which is owned by German automaker BMW AG, had no immediate comment on Monday.