A gunman robbed an armored-car guard at gunpoint outside a Wal-Mart (search), then pepper-sprayed startled shoppers as he fled, forcing more than 20 people to seek medical treatment.

Six people were hospitalized with breathing problems and 16 others were treated at the store after the gunman sprinted through crowded aisles spraying customers and workers.

"He said, 'Excuse me, man,' as he went by me. Then he pulled out a cylinder and started spraying this reddish fog into the air behind him," shopper Bill Rose said.

The armored-car guard was leaving the store Monday when a man in a trench coat and a wig pointed a gun at his head. The guard dropped his bag, and the gunman sprayed him in the face with pepper spray, then grabbed the sack containing an undetermined amount of money, police said.

"He had this orangish substance all over his eyes, his nose, his mouth and his hands," shopper Erin Foreman said. "Employees started pouring milk on him."

The robber escaped through another exit and fled in a white van, which was later found abandoned.

The FBI was assisting in the investigation in Burleson (search), about 40 miles southwest of Dallas.