US auto safety regulators have demanded additional information about Tesla's Autopilot, threatening civil penalties on the automaker for inadequate response, according to a document request reviewed Thursday by AFP.

The July 3 information request from the National Highway Traffic Safety Administration seeks more details about Tesla's modifications to the driver-assistance system, which has been probed by the agency since 2021 over its safety record.

"Failure to respond promptly and fully" could lead to civil penalties of up to $26,315 per violation per day for a maximum of $131.6 million, said the letter from Tanya Topka, acting director of the Office of Defects Investigation, which set July 19 as a deadline.

NHTSA launched the probe in August 2021 following a series of accidents involving Autopilot and emergency vehicles. After the initial inquiry, the agency expanded the investigation in June 2022.

The probe "aims to explore the degree to which Autopilot and associated Tesla systems may exacerbate human factors or behavioral safety risks by undermining the effectiveness of the driver's supervision," NHTSA has said.

In the July 3 letter to Tesla Director of Field Quality, Eddie Gates, Topka asked for "all modifications or changes" from the start of production for vehicles produced between 2014 and 2023.

This includes: the date of the modification; the reason for the change; and the primary means of distribution and whether the change required personal servicing.