Local WFAA is reporting that there is a 1,000 foot perimeter being set up outside of the store.
Update: More from the WFAA story:
Police were searching the area for a vehicle that had explosives in it.
SWAT members were already at the scene for the art event.
Rowlett/Sachse Scanner reported on Facebook that a suspect was inside a nearby Walmart, off Garland Avenue and Naaman Forest Boulevard, with a hand grenade.