A US government agency has formally opened a probe into Tesla's so-called Autopilot system following a spate of well-publicised crashes over the past few years. The investigation covers over three-quarters of a million vehicles, which has got to be a decent chunk of the US inventory shifted by Tesla since the start of the 2014 model year. It is estimated that in past three years alone, Tesla has sold a combined 430,592 units of Model X, Model S, and Model 3 in the United States. The National Highway Traffic Safety Administration's Office of Defects Investigation (ODI) said it was looking into the "level 2" Advanced Driver Assistance System (ADAS), which it said consists of lane-keeping and cruise control functions. "Most incidents took place after dark and the crash scenes encountered included scene control measures such as first responder vehicle lights, flares, an illuminated arrow board, and road cones," the agency said in a statement [PDF] today.