The NHTSA reaffirmed Tesla's 5-star safety rating late Monday. While the safety rating is important, it is based on crash tests -- not fires.
The agency has not replied to a request for comment. Tesla declined to elaborate on its statement.
Some traders on StockTwits, a social investing site, pointed out what they saw as misguided enthusiasm.
"$TSLA goes up on a months-old safety rating that was 'reiterated,'" said BullHorns. "Every company should reiterate positive news daily. LOL."
StockTwits user Fiftyfifty said that investors may be overreacting: "Knew people would confuse these ratings with the unrelated & ongoing investigation."
Tesla, in its press release, said it is "worth noting that a Tesla vehicle is over five times less likely to experience a fire than the average gasoline car and that there have been zero serious injuries or deaths for any reason ever, fire or otherwise, in a Model S."
--CNNMoney's Peter Valdes-Dapena contributed to this report.