(CNN) — At least six people were shot dead Tuesday night inside a Walmart in Chesapeake, Virginia, in an attack that unfolded within an hour of the store’s scheduled closing time, city officials said early Wednesday.

The shooter is dead, city officials said.

Investigators were sweeping the store overnight into Wednesday to search for victims or people who may have been hiding, Chesapeake Police spokesperson Leo Kosinski told CNN earlier. Police earlier reported people injured.

Investigators believe the shooter was an employee or former employee of the store who opened fire on other employees in a break room, a law enforcement source told CNN.

Officials believe the shooter at some point turned the gun on himself, according to the source.

Officers responded to the Walmart around 10:12 p.m. — less than an hour before it was set to close — and found victims and evidence of a shooting, Kosinski said.

Five patients were being treated at Sentara General Hospital in nearby Norfolk, Virginia, a spokesperson for Sentara Healthcare told CNN affiliate WTKR. An update on their conditions was not immediately available.

A news conference was scheduled for 8 a.m. Wednesday, Chesapeake city officials said on Twitter.

The shooting, which came two days ahead of Thanksgiving as customers were doing last-minute shopping, is yet another instance of how gun violence erupts in American life in places traditionally seen as safe, from schools to stores and even hospitals.

At the University of Virginia in Charlottesville — which is about 170 miles west of Chesapeake — a 22-year-old student allegedly opened fire on fellow students this month, killing three of them on a bus returning to campus from a field trip to Washington, DC.

Last weekend, a 22-year-old shot and killed five people at an LGBTQ nightclub in Colorado Springs, Colorado, and 19 others were injured, authorities said.

Tuesday’s shooting happened as the US has recorded more than 600 mass shootings so far this year, according to the Gun Violence Archive. The archive and CNN tally a mass shooting as one involving at least four killed or injured, not including the shooter.

Mother in shock after surviving shooting

Joetta Jeffery told CNN her mother, Betsy Umphlett, sent her texts from inside the store during the shooting, alerting her that someone had opened fire.

“I’m crying, I’m shaking,” Jeffery told CNN. “I had just talked to her about buying turkeys for Thanksgiving, then this text came in.”

Jeffery said her mother is uninjured but in shock, and they’ve been reunited.

Chesapeake city officials have asked people to stay away from the store during the investigation.

“Our first responders are well-trained and prepared to respond; please give them space to do so,” the city said in a tweet.

A reunification center was set up at the Chesapeake Conference Center, city officials said. They are asking that only immediate family and emergency contacts for people who were in the store go to the center.

In a statement, Walmart said it’s shocked at the tragedy that unfolded in one of its stores.

“We’re praying for those impacted, the community and our associates. We’re working closely with law enforcement, and we are focused on supporting our associates,” the company said in the statement.

The Washington, DC, field office of the Bureau of Alcohol, Tobacco, Firearms, and Explosives is assisting local police in the investigation, the bureau said on Twitter.