Tesla, the world's most valuable automaker by far, and its charismatic and brash chief executive, Elon Musk, have said Autopilot is not flawed, insisting that it makes cars much safer than others on the road. They have dismissed warnings from safety experts and the National Transportation Safety Board that have been critical of how the company has designed Autopilot. The company and Mr. Musk, who comments frequently on Twitter, did not respond to messages seeking comment on Monday and issued no public statements about the new investigation.