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.
• Tesla did not immediately respond to Axios' request for comment Friday.

Zoom in: NHTSA also took aim at the "multipart remedy" Tesla deployed to fix the issue.

• "A portion of the remedy both requires the owner to opt in and allows a driver to readily reverse it," the regulator noted.
• Tesla also deployed software updates that appear related to the recall's concerns but were not formally made part of the recall.
• "This investigation will consider why these updates were not a part of the recall or otherwise determined to remedy a defect that poses an unreasonable safety risk," NHTSA said.

The big picture: A Washington Post investigation last year found that autopilot feature was enabled when it shouldn't have been in at least serious eight car accidents, some of which included fatalities.

• The Justice Department opened its own probe into Tesla's autopilot technology last year.
• NHTSA said in 2022 that Teslas made up 70% of the car crashes that involved advanced driver assistance systems in the past year.

Our thought bubble, from Axios' Nathan Bomey: This is the latest example of Tesla sparring with regulators about Autopilot, which critics say promises more than it can deliver.

Go deeper: Tesla stops the bleeding — but not the questions