WASHINGTON - 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 identified concerns due to crash events after vehicles had the recalled 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."

(Reporting by David Shepardson; editing by Jason Neely)