Federal safety investigators want Tesla to tell them how it developed the fix in a recall of more than 2 million vehicles with the company’s Autopilot partially automated driving system. Investigators with the U.S. National Highway Traffic Safety Administration have concerns about whether the recall remedy worked because Tesla has reported 20 crashes since it was issued in December. In a letter to Tesla, investigators wrote that they could not find a difference between warnings to the driver to pay attention before the recall and after the recall fix was sent out in an online software update. Read moreUS seeks information from Tesla on how it developed and verified whether Autopilot recall worked