US asks Tesla why it didn't recall Autopilot driving system

In a letter sent to Tesla on Tuesday, the National Highway Traffic Safety Administration told the electric car maker that it has to do a recall if an over-the-internet update mitigates a safety defect. 

The agency also ordered Tesla to provide information about its “Full Self-Driving” software that's being tested on public roads with some owners. The latest clash provides more evidence of escalating tensions between Tesla and the agency that regulates partially automated driving systems. 

In August the agency opened an investigation into Tesla's Autopilot after getting multiple reports of vehicles crashing into emergency vehicles with warning lights flashing that were stopped on highways. The letter was posted on the NHTSA website early Wednesday.