Chopper 6 was over the scene just before 8 p.m. as officers responded to the Walmart along North MacDade Boulevard in Darby Township.
According to investigators, two men were arguing inside the store when one of them pulled out a gun and fired between five and 10 shots.
A female Walmart employee was struck in the calf. She was taken to Penn Presbyterian Medical Center in Philadelphia with non-life-threatening injuries.
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The gunman fled the scene and remained at large Friday night.
One woman working as a DoorDash driver said she was in a nearby store when people were told to get on the ground.
"The cops came over with their rifles and told everyone in the store to get down. I had no clue what happened," she said. "It's crazy. You can't even shop anymore. It's ridiculous."
Police say multiple people were briefly detained by police, but only for information.
Walmart employees were told not to speak to reporters, but Action News did talk to a former employee. She heard the news from her friends who still work there.
"They're in shock. They trained for this stuff, but you would never think it would happen," the woman said.
Walmart said in a statement that the safety of their associates and customers is a top priority.
"We're grateful to first responders for their swift response and are working closely with law enforcement as they continue their investigation," a spokesperson said.
The Delaware County District Attorney's Office says it's working to identify the shooter, but reiterated that there's no reason to believe the public is in danger.
Anyone with information is asked to contact Darby Township police.