GENEVA -- Police in southern Switzerland say they are investigating the death of a German man after a Tesla car he was driving crashed into a guardrail and burst into flames last week.

The Bellinzona city fire brigade reportedly pulled from its Facebook page a comment that suggested the vehicle's lithium ion batteries could have triggered a phenomenon involving a "rapid and unstoppable increase in temperature" before Thursday's accident.

Reached by phone on Tuesday, the fire brigade said simply that an investigation was underway and declined to comment further.

Bellinzona police said the German driver, 48, crashed while driving on the highway in circumstances that have not yet been clarified.

Tesla said in a company release that it's saddened by the crash and is fully co-operating with authorities to find out the facts. "Tesla has not yet received any data from the car and thus does not know the facts of what occurred, although it appeared to be a high-speed collision," the statement said.

The U.S. National Transportation Safety Board is investigating at least two recent fires involving Tesla vehicles. Last week the agency opened a probe of a Model S that caught fire after crashing into a wall in Florida. Two 18-year-olds were trapped in the vehicle and died in the flames.

The U.S. agency also is looking into the performance of the company's semi-autonomous Autopilot system after a Tesla Model X SUV crashed into a barrier on a California highway. The driver in that incident died, but Tesla said he was pulled from the vehicle before it caught fire.

Police in Utah are looking into a collision involving a Tesla electric car that hit a fire department vehicle over the weekend. Drive told investigators she had the vehicle's semi-autonomous Autopilot mode engaged when she slammed into the back of the truck.