Police in Amarillo were responding to reports on Tuesday of shots being fired and an armed subject possibly holding hostages at a Walmart store in the Texas panhandle city.
There have been no reports of injuries in the incident, the City of Amarillo said on its Twitter feed.
Amarillo police did not offer any further details on the suspect or what caused the situation.
Traffic in the vicinity has been halted and the Walmart has been shut down, the Randall County Sheriff's Office said, urging people to stay away from the area.
"There are reports the subject inside may have hostages," it said in a Facebook post.
Police have been evacuating people from the building, Amarillo TV broadcaster ABC 7 reported.