Police have not identified the driver or the victims, although Ludwig said none of them are children.
Ludwig told the Register that the pickup driver was an older man. He was alone in the truck and among the two people injured.
In a statement to WHO-TV, a Walmart spokesperson said that the company was "heartbroken by what appears to be a tragic accident."
"Our focus right now is on our associates, customers and gathering information," the statement said. "We will continue working closely with law enforcement and emergency responders."
Pella is about 40 miles southeast of Des Moines.