The National Highway Traffic Safety Administration is investigating whether Tesla did enough to improve driver safety after recalling roughly 2 million vehicles to fix a software defect in their autopilot monitoring system.

Why it matters: This is just the latest hurdle Tesla has faced over safety concerns related to its autopilot software. NHTSA has been investigating the company's autopilot system and crashes for years.

Driving the news: The original recall, issued last December, covered almost all of the cars Tesla had sold in the U.S. at the time.

It was intended to install a software update to monitor and ensure that drivers were paying attention while using the vehicle's autopilot system.

The feature allows Teslas to automatically steer, accelerate and brake within their lane. An advanced version of it can enable the vehicle to change lanes on the highway.

However, NHTSA noted that its regulators have "concerns due to post-remedy crash events and results from preliminary NHTSA tests of remedied vehicles," according to documents released Friday.