WASHINGTON (Reuters) -U.S. auto safety regulators said Friday they have opened an investigation into whether Tesla's recall of more than 2 million vehicles announced in December to install new Autopilot safeguards is adequate.

The National Highway Traffic Safety Administration (NHTSA) said it was opening an investigation after the agency identified concerns due to crash events after vehicles had the recall software update installed "and results from preliminary NHTSA tests of remedied vehicles."

NHTSA also cited Tesla's statement "that a portion of the remedy both requires the owner to opt in and allows a driver to readily reverse it."

The agency said Tesla has issued software updates to address issues that appear related to its concerns but has not made them "a part of the recall or otherwise determined to remedy a defect that poses an unreasonable safety risk."

Tesla said in December's its largest-ever recall covering 2.03 million U.S. vehicles - or nearly all of its vehicles on U.S. roads - was to better ensure drivers pay attention when using its advanced driver assistance system.

The new recall investigation covers Model Y, X, S, 3 and Cybertruck vehicles in the U.S. equipped with Autopilot produced between the 2012 and 2024 model years, NHTSA said.

Tesla said in December Autopilot's software system controls "may not be sufficient to prevent driver misuse" and could increase the risk of a crash.

The auto safety agency noted Friday that during its Autopilot safety probe it first launched in August 2021 it identified at least 13 Tesla crashes involving one or more death and many more involving serious injuries in which "foreseeable driver misuse of the system played an apparent role."

Tesla did not immediately respond to a request for comment.

In February, Consumer Reports, a nonprofit organization that evaluates products and services, said its testing of Tesla's Autopilot recall update found changes did not adequately address many safety concerns raised by NHTSA and urged the agency to require the automaker to take "stronger steps," saying Tesla's recall "addresses minor inconveniences rather than fixing the real problems."

Tesla's Autopilot is intended to enable cars to steer, accelerate and brake automatically within their lane, while enhanced Autopilot can assist in changing lanes on highways but does not make vehicles autonomous.

One component of Autopilot is Autosteer, which maintains a set speed or following distance and works to keep a vehicle in its driving lane.

Tesla said in December it did not agree with NHTSA's analysis but would deploy an over-the-air software update that will "incorporate additional controls and alerts to those already existing on affected vehicles to further encourage the driver to adhere to their continuous driving responsibility whenever Autosteer is engaged."