There were no reports of injuries at the Farm King store in the western Illinois city of Macomb, State Police Lt. Harry Douglas said.
Authorities said the gunman was still inside the store Wednesday afternoon, and a SWAT team had surrounded the building.
Two women, two men and a small child were released from the store apparently unharmed at about 3:30 p.m. and got into an ambulance, the (Peoria) Journal Star newspaper reported. Two other people left the building earlier in the afternoon.
Shortly before the five people were released, Macomb Police Chief Curt Barker said an unknown number of customers and employees were believed to be inside the store. Police spokesmen weren't immediately available to comment after their release.
Police received a 911 call just before 1 p.m. Wednesday from a Farm King employee who said a man with a rifle was inside the store, and when police arrived someone shot at them from inside, Barker said.
In addition to SWAT teams and police, a large truck with "Homeland Security" on its side also arrived at the scene.
Witness Richard Moulton was at Farm King shopping for a battery charger with his father when they suddenly heard someone arguing in another aisle and then yelling.
"I heard him say to a guy, 'Get away from me if you don't want to get shot,"' Moulton said. "I couldn't make out what they were arguing about, but I said to my dad, 'Somebody sounds really mad back there."'
Seconds later, four or five shots ran out, and an employee came around the counter, wide-eyed, to say that there was a man with a loaded gun, Moulton said.
"We started looking for a way to get out really fast," he said.
He and his father bolted out of a rear door and into a parking lot, hiding behind a snow bank. He said police arrived within minutes and surrounded the building.
Employees in nearby businesses huddled inside offices, peering through windows at the scene of the standoff.
"From my windows, I can see half a dozen state trooper cars and Macomb police - and guys squatting with guns behind it," said John Gentry, the owner of Gentry Motor, which is about 100 yards away from the farm supply store.
Farm King employees in red uniforms could be seen leaving the area as police arrived earlier in the afternoon, Gentry said. He added that he wasn't concerned for his own safety and "that police appear to have it under control."
McDonough District Hospital dispatched ambulances to the scene as a precaution, said John Maguire, director of public relations.
Western Illinois University alerted its faculty and staff to avoid the street where the store is located. The campus remained open.
Macomb is 183 miles west of Chicago.
