The letter, which came after NHTSA opened a probe into Tesla’s Autopilot on Monday, added to pressure on Tesla.
Tesla did not immediately respond to a request for comment.
Autopilot is a standard feature for Tesla cars and enables the vehicles to maintain distance from cars in front. Tesla sells its advanced driver-assist features such as lane changing and automated parking under the name Full Self-Driving for $1,000, although the system does not make its vehicles fully autonomous.
Musk, who has nearly 60 million Twitter followers, uses the term FSD frequently, generally referring to the Tesla package of features. Musk has touted how safe the technology is and promised that its vehicles would soon drive themselves, only to miss his own deadlines.
NHTSA said in June that since 2016 it has opened 30 investigations into Tesla crashes in which the agency suspects advanced driver assistance systems were in use.
NHTSA in 2018 said in a letter to Tesla the company had made “misleading statements” about the safety of its Model 3 and had confused consumers. The agency referred the issue to the FTC to investigate whether Tesla’s statements constituted “unfair or deceptive acts or practices.”
