Elon Musk’s boasts about the safety of Tesla’s vehicles earned him a slap on the wrist from US regulators last year, newly released documents show.

The National Highway Traffic Safety Administration sent the Tesla chief executive a cease-and-desist letter last October asking him to stop saying Teslas are the safest cars in the world.

The NHTSA letter, which was published by document website PlainSite on Wednesday, said that Tesla made “a number of misleading statements” on its company blog about the safety of its vehicles, including that the NHTSA’s tests of the Model 3 sedan showed that “it has the lowest probability of injury of all cars the safety agency has ever tested.”

The NHTSA, which gave the Model 3 the top rating on the agency’s 5-Star Safety Ratings Program, called Tesla’s claims “inaccurate,” explaining in its letter that the nature of its crash tests makes it “impossible” to compare the results of vehicles that vary in weight by more than 250 pounds.

“The guidelines warn against comparison statements like these because such statements mislead consumers about the relative safety of different vehicle models,” NHTSA Chief Counsel Jonathan Morrison said in the letter.

The NHTSA said it was referring Tesla’s safety claims about the Model 3 to the Federal Trade Commission to investigate whether the automaker’s statements constituted unfair or deceptive acts or practices.

An FTC spokesman said only that “investigations are non-public, and we don’t comment on … the existence of an investigation.”

Tesla has still not changed the blog post that says its autos “are engineered to be the safest cars in the world.”

Asked about the documents, a Tesla spokesman on Wednesday pointed to statements in an Oct. 31, 2018, letter it sent to the NHTSA, in which it defended itself by saying the Model 3 had the lowest risk of occupant injury of any vehicle in US government tests.

In that letter, the company argued that its statement was “neither untrue nor misleading.”

“We had hoped NHTSA would welcome such an achievement because it was presented in an objective manner using the agency’s own data,” Tesla said.

Tesla shares rose 1.2%, closing at $233.42.

With Reuters