Some of the complaints indicate that the vehicle was operated in electric mode for awhile after the gas engine stalled, NHTSA said. In at least half the cases, the car wouldn't restart and had to be towed.
Published reports detailing the potential problem first surfaced earlier this month, but NHTSA, while looking into the complaints, had yet to formalize an investigation until now.
There have been no reported cases of injuries.
The Toyota Prius currently dominates the competition, making up 64% of the overall hybrid market in 2004. Prius sales tripled in April to
11,345, moving towards the company's target of 300,000 a year.
Toyota previously pegged its long range annual forecast at 1 million hybrid vehicles.
Last month, Toyota announced one of its biggest ever recalls with
790,000 trucks and sport utility vehicles being called back to shops due to problems with the front suspension. See full story.
U.S.-listed shares of Toyota Motor (TM: news, chart, profile) added 1% to $72.44 in early trading.