The Phoenix Fire Department concluded its investigation of the Nikola Tre battery electric truck fires that happened last month and there is no evidence of arson, according to Electrek, which got a response from Daniel Cheatham, the City of Phoenix’s Fire Investigations Task Force division chief.

In an email sent to the online publication, the fire department said that the cause of the fire at Nikola’s headquarters was officially declared to be “undetermined.”

“It officially is undetermined,” said Cheatham. “But we do not have any evidence at all that supports arson.”

When the news broke about the Class 8 semi trucks catching fire outside the firm’s Phoenix headquarters, Nikola said in a statement on Twitter that it suspected “foul play,” as a vehicle was seen in the area before the vehicles started smoking and ultimately flaming.

Gallery: Nikola Tre BEV

At the same time, the Phoenix Police Department’s investigation is still open, with detectives analyzing evidence such as videos and witness tips.

“Overall, the investigation will continue as detectives work with insurance companies as they determine any and all outcomes,” a police spokesperson replied to an inquiry from Electrek. “When arson investigators complete their investigation they will determine if criminal behavior took place or not.”

With this being said, internal documents revealed late last month point to a possible issue with the truck’s battery pack, where laser welding could puncture some cells of a module, causing the cells to corrode after just two test cycles and potentially generating excess heat.

Nikola told InsideEVs in a statement that packs with weld issues did not make it into the production semi trucks, but that both its internal engineering team and a third-party team have initiated investigations to determine the root cause of the fires, and that the results will come out in several weeks.

The Iveco-based Class 8 Nikola Tre semi is made in Coolidge, Arizona, and has an estimated driving range of 330 miles thanks to a massive 733-kilowatt-hour battery pack that can charge to 80 percent in 90 minutes at 350 kW.

Got a tip for us? Email: tips@insideevs.com