The shooter was described by alleged witnesses on social media as one of the store managers. Via CNN:

The gunman who killed six people at a Walmart in Virginia on Tuesday night was an employee at the store, Chesapeake Police Chief Mark Solesky said Wednesday. Solesky said the gunman is believed to have died from a self-inflicted gunshot wound, but his identity has not been released because his next of kin has not been notified.

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 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.