The gunman, identified as Andre Bing, 31, of Chesapeake, Virginia, said nothing as he began firing on workers gathered late on Tuesday ahead of their overnight shift, according to two employees who were in the break room.
"I just watched three of my coworkers/friends be killed in front of me," Donya Prioleau wrote in a Facebook post.
"Andre killed them in cold blood ... I cannot unsee what happened in that break room."
At least four others were wounded in the attack, which took place while about 50 people were inside the store.
Chesapeake police identified the dead as Brian Pendleton, 38, Kellie Pyle, 52, Lorenzo Gamble, 43, and Randy Blevins, 70, all from Chesapeake, and Tyneka Johnson, 38, of neighbouring Portsmouth.Â
A 16-year-old boy from Chesapeake, whose name was withheld because he was a minor, was also killed.
Authorities say they were still investigating what may have motivated Bing, who came armed with a single handgun and multiple magazines of ammunition.
The city said its SWAT team had executed a search warrant at his house.
"Detectives are working to learn more about the shooter's background and what may have motivated the shooting," the city said in a statement.
Another Walmart employee, Briana Tyler, told ABC's Good Morning America: "I looked up and my manager just opened the door and he just opened fire."
Several of Bing's co-workers told CNN he had exhibited strange and sometimes threatening behaviour in the past.
He also made paranoid comments, expressing concerns the government was monitoring him, they told CNN.
Coming on the heels of the killing of five people at a Colorado Springs LGBTQ nightclub on Saturday, the latest massacre prompted a fresh round of condemnations by public officials and calls by activists for tighter gun control.
US President Joe Biden on Wednesday called the shooting "yet another horrific and senseless act of violence", noting a shooting earlier this month that left three University of Virginia students dead.
"There are now even more tables across the country that will have empty seats this Thanksgiving," he said in a statement.
Bing worked for the company since 2010, most recently as an overnight team leader at the Walmart Supercenter in Chesapeake, a city of about 250,000 residents just south of Norfolk.
Jessie Wilczewski told WAVY-TV she hid under a table and the shooter pointed the gun at her and told her to go home.
"It didn't even look real until you could feel the pow-pow-pow. You can feel it," the store employee said.
"I couldn't hear it at first because I guess it was so loud. I could feel it."
Tuesday's bloodshed marked the latest spasm of gun violence in the US, where an average of two mass shootings - defined as an incident killing or injuring four or more people - occur every day, according to GunViolenceArchive.org.
