Tesla Inc. shares tumbled the most in six weeks after a formal rollout of the Cybercab driverless car left investors underwhelmed and drew fresh scrutiny from US auto regulators. The National Highway Traffic Safety Administration on Friday opened a probe into the process and technical data Tesla relied on when the company self-certified the vehicles — which have no steering wheels or foot pedals — as compliant with all federal safety standards. The agency conducts investigations when “certified vehicles appear to not adhere to these requirements,” it said in a statement. The decision underscored the uncertain path forward for the unorthodox vehicle, which lacks certain driver controls that are typically required by federal standards. The regulatory hurdles are among the many unknowns surrounding the vehicle, alongside technological and market-readiness questions. The Cybercab, which has been in testing in multiple US cities in recent months, is seen as a key piece of Tesla’s plan to evolve beyond a maker of electric vehicles into a pioneer of artificial intelligence products. Chief Executive Officer Elon Musk has pitched Cybercab as the eventual backbone of a vast robotaxi fleet reaching most of the US, vowing to deploy thousands of the cars. Against that backdrop, the company’s introduction of the vehicle into passenger service late Thursday in Austin was uncharacteristically low-key, with no livestream or media access. The automaker opened a Cybercab-specific webpage earlier in the day and posted on social media that public rides would begin Friday evening. The “Cybercab event was largely a bust,” Gary Black, co-founder of Future Fund Advisors, wrote on X. “The launch offered little detail and key questions remained unanswered.” Tesla shares fell 6% at 11:09 a.m. Friday in New York, the biggest intraday since July 23. Details of the closed-door launch were posted by influencers who were invited.