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After an out-of-control Tesla Model S plowed into a stand of palm trees on a highway median outside Fort Lauderdale last month, police rushed to put out the ensuing blaze using a department-issued fire extinguisher. It was a wasted effort. The car kept on burning after the crash, which killed the driver.

The police may not have known lithium-ion batteries inside electric vehicles, once ignited, can’t be put out with chemicals from a conventional extinguisher. The battery fires are susceptible to a self-destructive chain reaction known as thermal runaway, causing a feedback loop of rising temperatures. The Tesla fire stumped a series of first responders in Florida. Firefighters eventually doused the flames with water, which seemed to work, but the wrecked car reignited twice more after being towed away. That prompted what a police report later termed “extraordinary measures,” including a call to Broward County’s hazmat unit for advice on stamping out the fire once and for all, Bloomberg reports.

The accident illustrates the challenges faced by first responders unfamiliar with the special characteristics—and hazards—of electric vehicles’ powertrains. Safety experts say the only way to extinguish a lithium-ion battery inside a car is with thousands of gallons of water, much more than what it takes to stop a fire in a typical gasoline engine. The other option is to just let it burn itself out. “It’s such a difficult fire because it takes so much water to put out,” said Robert Taylor, fire marshal in Davie, Fla., where the crash occurred.

Tesla also regularly meets with first responders and donates its cars for training purposes. But there’s only so much that can be done once battery cells begin to spontaneously explode. The company’s online emergency response guide notes: “Battery fires can take up to 24 hours to extinguish. Consider allowing the battery to burn while protecting exposures.”