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For one day in August, a D.C. fire department ambulance seemed like the most dangerous place in town. At the height of labor tensions with management and concerns over the department’s shoddy upkeep schedule, two ambulances burst into flames. Was it really just poor maintenance, or something more sinister?

Paul Quander, the city’s deputy mayor for public safety, leaned toward the latter. Quander asked the Metropolitan Police Department to investigate the fires and make sure nothing “untoward” was going on. The implication: Union firefighters, knowing anything that made Fire Chief Kenneth Ellerbe look bad would help them, had set fire to their own vehicles.

MPD’s investigation, though, pinned the fires on mechanical problems, finding no proof that arsonists had been at work. The report seemed comforting—the District’s firefighters hadn’t turned into firebugs. But it meant something potentially more ominous: The District’s emergency vehicles, apparently, could spontaneously combust all on their own.