WASHINGTON—The National Highway Traffic Safety Administration said Tuesday it is opening a new special crash investigation into a fatal accident in California involving a 2018 Tesla Model 3 where advanced driver assistance systems are suspected of having been used.

Since 2016, NHTSA has opened more than three dozen Tesla special crash investigations in cases where advanced driver assistance systems such as Autopilot were suspected of being used, with 20 crash deaths reported.

Tesla did not immediately respond to a request for comment.

This is the first new special crash investigation open since March. In that case, the NHTSA said it was opening a new special investigation into a February fatal crash in California involving a Tesla (TSLA.O) Model S, in which an advanced driver assistance system was suspected of having been used.

The agency is investigating the crash of a 2014 model year Tesla involving a fire truck in Contra Costa County, California. The fire department said a Tesla struck one of its fire trucks and the Tesla driver was pronounced dead at the scene.

In December, NHTSA said it had opened two new special investigations into crashes involving Tesla vehicles where advanced driver assistance systems are suspected to have been in use.

NHTSA typically opens more than 100 special crash investigations annually into emerging technologies and other potential auto safety issues that have, for instance, previously helped to develop safety rules on air bags.

In June, the federal agency upgraded to an engineering analysis its defect probe into 830,000 Tesla vehicles with driver assistance system Autopilot and involving crashes with parked emergency vehicles including fire trucks. That step was necessary before the agency could demand a recall.