The NHTSA's investigation will now look into whether Tesla deemed that certain parts of its self-certification process were 'are inapplicable to the Cybercab.'

Tesla's Cybercabs are carrying paying passengers in Austin, even as federal regulators investigate whether the vehicles comply with federal safety standards. Why it matters: Texas authorizes companies to operate driverless vehicles without approving each vehicle model.

Enthusiasm from fans at the event contrasted with investor concerns about deployment plans and new regulatory questions.

Tesla shares fell Friday after the electric-vehicle maker's much-anticipated Cybercab launch, leaving some Wall Street analysts with more questions than answers. The stock dropped more than 6%, putting it on pace for its worst day since July 23 — when it plunged 15%.

Riders in Austin can now hail the two-seater. The company's own July letter says batteries still set the pace.