An apparent explosion and fire at the world's largest oat mill in Iowa sent thick black smoke billowing over downtown Cedar Rapids Saturday.

What we know: The Cedar Rapids Fire Department says it happened at 4:44 p.m. local time and when first responders arrived they found heavy smoke and flames coming from the north side of the building.

Additional fire crews from Marion, Hiawatha and Linn County were called in to help battle the blaze, which was extinguished around 6:25 p.m. What they're saying: Cedar Rapids Mayor Tiffany O'Donnell said in a Facebook video that all workers were safe and accounted for.