Three day's later, An air elemental starts wreaking havok near the outskirts of town. This one isn't a fluke. It's been summoned by a cabal of elementalists to assassinate the cheif magistrate.
The cabalists strike again two days later, this time sending a water elemental to kill the head of the merchant's guild.
Two days after that, the cabal sends an earth elemental to kill the captian of the city guard.
After fighting the four elementals, your players think they're out of hot water, but they're not. The first fire elemental was a fluke, drawn to the scene by the burning building. And two days later, the cabalists send a fire elemental to attack the mayor, while simultaniously attacking in person those targets that the PCs have managed to save.