WASHINGTON U.S. auto safety officials said on Thursday they are investigating two additional reports of General Motors robotaxi unit Cruise self-driving cars engaging in inappropriately hard braking and being struck by other vehicles.

The National Highway Traffic Safety Administration (NHTSA) said in December it had opened a formal safety probe into Cruise after reports of three crashes in which Cruise vehicles were struck from behind by other vehicles after the autonomous vehicles braked quickly resulting in two injuries.

NHTSA said in an Oct. 20 letter made public Thursday it had two additional crash reports involving Cruise vehicles that braked "with no stated obstacles ahead" and was seeking additional information by Nov. 3.

(Reporting by David Shepardson; Editing by Chris Reese)