CEDAR RAPIDS — A spate of fires set by tossing Molotov cocktails and intentionally setting piles of derecho debris ablaze led to a spike last year in arsons — a crime that’s typically hard for authorities to solve. According to data from the Cedar Rapids Police Department, there were 32 fire incidents in 2020 determined to be arson. That’s a 60 percent increase from the 20 cases of arson reported the year before. Twelve of those incidents, according to Cedar Rapids Fire Marshal Vance McKinnon, were fires sparked from Molotov cocktails or fires involving debris created by the Aug. 10 derecho. “If we remove those incidents from the 2020 total, we would be right even with where we were the year before,” he said.