Six days after a fiery crash on Highway 101 involving a Tesla Model X took the life of a 38-year-old San Mateo man, the car’s high-voltage lithium-ion battery re-ignited while sitting in a tow yard, according to the Mountain View Fire Department.

The revelation was included in a safety memo written last month by Mountain View Fire Chief Juan Diaz, and first reported Wednesday by KTVU2.

Thursday, Diaz told this newspaper that he issued the memo on April 5 to protect firefighters in his department when dealing with future electric vehicle fires. When neighboring departments contacted Mountain View regarding the Tesla crash for lessons learned, he shared the memo.

“We talked about the challenges we faced,” Diaz said. “This is a high voltage direct current that could result in a lethal electrical shock.”

The National Transportation Safety Board is investigating Highway 101 crash and three other accidents also involving Teslas, including a fiery 2014 Model S crash Tuesday in Florida that killed two teenagers. Also under investigation: A Model S crashed into a fire truck near Culver City in January, and the driver reportedly said Autopilot was engaged at the time. And it is looking into a battery fire of a Model X that drove into a home’s garage in Lake Forest in August.

The Mountain View Fire Department safety report detailed issues Mountain View firefighters faced March 23 when a Tesla Model X crashed into a concrete barrier on southbound 101 near the Highway 85 flyover ramp. The driver, Walter Huang, was killed in the crash. The car’s battery suffered unprecedented damage, according to Tesla.

On the company website, Tesla wrote “the reason this crash was so severe is that the crash attenuator, a highway safety barrier which is designed to reduce the impact into a concrete lane divider, had either been removed or crushed in a prior accident without being replaced.

“We have never seen this level of damage to a Model X in any other crash,” the company wrote in a blog post.

Tesla also reported that the vehicle’s autopilot function was active at the time of the crash.

Diaz said his department has dealt with several electric vehicle fires but never an “accident of this magnitude.”

In the memo, Diaz said the damaged ion-battery remained energized after being doused with water and that the damaged battery presented a risk of electric shock “as many of the cells and high voltage wires were exposed.” He also stated that “the short-circuit event that had occurred when the battery’s interior was breached in the collision, the battery cells continued to generate heat in a process called “thermal runaway.”

During a thermal runaway event, temperatures can exceed over 900 degrees Fahrenheit, Diaz said.

“That’s what we experienced at the scene,” Diaz said. “That’s why we called the manufacturer of the vehicle.”

On the day of the crash, Tesla engineers dismantled about 25 percent of the battery. About five-and-a-half hours after the crash, the fire department determined the Model X was safe enough to be towed away. Mountain View firefighters escorted the tow truck to the yard, in case the battery re-ignited.

The battery reignited twice in the storage yard within a day of the accident and again six days later on March 29. Two weeks later, in an effort to avoid more fires, the NTSB and Tesla performed a battery draw down to fully de-energize it, Diaz wrote in the memo.

“We are very familiar and aware that a lithium ion battery that has been damaged has the potential to re-ignite,” Diaz said. “The battery overheats. It’s not an event that surprised anybody.”

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Tesla driver ‘completely reclined’ and apparently asleep at 90 mph, police say On the company website, Tesla wrote that its battery packs “are designed so that in the rare circumstance a fire occurs, it spreads slowly so that occupants have plenty of time to get out of the car. According to witnesses, that appears to be what happened here as we understand there were no occupants still in the Model X by the time the fire could have presented a risk. Serious crashes like this can result in fire regardless of the type of car, and Tesla’s billions of miles of actual driving data shows that a gas car in the United States is five times more likely to experience a fire than a Tesla vehicle.”

The crash remains under investigation by the NTSB.