A Dec. 31 tweet suggested drivers with more than 10,000 miles using Tesla's "Full Self-Driving" (FSD) software system should be able to disable the "steering wheel nag," an alert that instructs drivers to hold the wheel to confirm they are paying attention.The auto safety agency confirmed the questions about Musk's tweet are in connection with its ongoing defect probe into 830,000 Tesla vehicles with driver assistance system Autopilot and involving crashes with parked emergency vehicles.NHTSA is reviewing whether Tesla vehicles adequately ensure drivers are paying attention, and previously said evidence suggested drivers in most crashes under review had complied with Tesla's alert strategy that seeks to compel driver attention, raising questions about its effectiveness.Tesla sells the $15,000 FSD software as an add-on which enables its vehicles to change lanes and park autonomously.Since 2016, NHTSA has opened more than three dozen Tesla special crash investigations where advanced driver assistance systems such as Autopilot were suspected of being used with 19 crash deaths reported."