The US National Transportation Safety Board opened an investigation into the recent crash and fire of a Tesla Inc. Semi truck in what is believed to be the first probe involving the company’s electric big rig.

The Tesla Semi ran off the road on Interstate 80 near Emigrant Gap, California, around 3 a.m. Monday. No one was injured, but the vehicle caught fire and ignited nearby brush, closing the freeway in both directions for several hours.

Tesla makes the Semi at its factory near Reno, Nevada, and uses the vehicle to transport parts to its auto plant in the San Francisco Bay area. Though the Semi was first unveiled in November 2017, it’s still in pilot production almost seven years later.

The NTSB said in a statement that it decided to investigate “due to its interest in the fire risks associated with lithium-ion batteries.” The Tesla truck’s batteries burned for about four hours, according to the statement.

The agency has long been focused on electric-vehicle blazes involving the batteries, in part because of the challenges emergency responders can have with extinguishing the fires.

Tesla’s shares were little changed at 9:31 a.m. Thursday in New York.