Harford County Sheriff's Office said there are "multiple victims" and the situation has not yet been contained. "It's very scary when it's just right there on your doorstep because you can see a billion news reports about shooters, about emergency instances, and it still doesn't make it real to you".
They have cleared and searched the 210,000 square foot facility for additional victims and information but believe the suspect acted alone.
The Harford County sheriff's office has said the woman was identified as a temporary employee, Snochia Moseley of Baltimore County.
The Rite Aid distribution center sits among a number of warehouses.
A Baltimore hospital said it had received four patients with gunshot wounds from the shooting.
Police have officially released the identity of the woman suspected to be the Maryland shooter.
"The first shot she took was more of a grazing wound and she actually shot herself a second time", Gahler said.
Governor Larry Hogan tweeted: "We are closely monitoring the horrific shooting".
The Rite Aid center, situated among warehouses in an industrial park, has almost 1,000 employees, company spokesman Pete Strella said.
Mike Carre, an employee of a furniture logistics operation next to the distribution centre, said he helped tend to a wounded man.
Workers at the distribution centre described terrifying moments of "crazy" gunfire and people screaming and running in all directions after the shooting. "The State stands ready to offer any support", he tweeted. She described it as a support facility adjacent to a larger building.
Gahler said she used a handgun that she legally purchased in March to fire a total of 13 rounds Thursday morning, and died after shooting herself in the head.
The three injured people are expected to survive.
Last year, three were killed and two injured in a shooting at a kitchen countertop company in Edgewood, Md., 11 miles from Thursday morning's attack. Hours later in western Pennsylvania, a gunman wounded four people at a court building before being killed by police.
"It's nearly like a ticking time bomb and when that mental illness manifests into violence", Harford County Executive Barry Glassman said at the news conference.