All the fires happened within a five-mile radius. All started alongside a road between 4 p.m. and 5 p.m. Investigators found firework debris at Sunday's fire. They're relying on witnesses to crack the case.
"It's very challenging when we have very little evidence, no leads and no witnesses. It just leaves us at a dead end," said Naches Heights Fire Chief Chris O'Dell.
The Yakima County Fire Marshal will look over all the fire reports to see how to pursue the case.