Several electric semi trucks caught fire at the Nikola building near 40th Street...

Several electric semi trucks caught fire at the Nikola building near 40th Street and Broadway early Friday morning.

Firefighters have been spraying water over the battery fire, but if they stop, they say the flames reignite. 

"So these cells that are packed in these batteries, they contain a lot of energy in a small amount of space, so extinguishing these are very difficult," said Capt. Todd Keller with Phoenix Fire. "[What] we're doing right now is that we're cooling these trucks down and preventing extension."

Typically, crews would put sand over this type of fire to extinguish it. They cannot do that in this case because the semis are so big, so they're spraying as much water as they can to cool down the vehicles.

Authorities say the process could take over 3 weeks to completely cool the vehicles.