AT least three people are dead after a shooting at a business park in the US during which staff texted their loved ones to tell them they loved them.

Harford County Sheriff Jeffrey Gahler confirmed there were "multiple wounded, multiple fatalities" and that the suspect was in a critical condition in a local facility.

Police refused to release details of the suspect, but multiple media outlets suggested the shooter may have been a woman.

Sheriff Gahler said police were called to a Rite Aid warehouse in Aberdeen, northeast of Baltimore, at 9.06am local time, after reports of shots fired.

He told a news conference it was believed a single weapon was used in the shootings - a handgun - and police did not believe there was any further threat to the community.

A law enforcement official with knowledge of the shooting, who wasn't authorised to discuss details and spoke on condition of anonymity, said three were dead, but stressed the number was based on preliminary information.

A woman whose mother works at the warehouse showed WBAL-TV a harrowing text her mum sent while hiding from the shooter.

"I love you both more than u will ever know," wrote Alexi Scharmann's mother in the message. "There is a shooter in the building. I am hiding. I love you.

"Be good an take care of daddy an the pets if something should happen."

Ms Scharmann said that while there was security and metal detectors in the adjacent building, there were not the same safeguards in the Liberty building, where her mother works.

"Everybody there is so tight-knit," she said. "There's not that many people that work there. Everybody is so nice over there, that's why it's a shock."

By 10.30am, her mother texted again to say she was outside and safe.

The FBI's Baltimore field office earlier tweeted that it was responding and assisting the sheriff's office with an "active shooter" situation.

Harford County Sheriff's Office warned on Twitter that the situation was "still fluid" and asked people to avoid the area.

Susan Henderson, a spokeswoman for drugstore Rite Aid, said authorities were at the scene and the company was cooperating with the investigation.

She said the shooting happened at a support facility adjacent to a larger building and roughly 1000 employees work there, but she understood the "location was secure".

The Baltimore field division of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives tweeted that its special agents were also responding.

Maryland governor Larry Hogan tweeted: "We are closely monitoring the horrific shooting in Aberdeen. Our prayers are with all those impacted, including our first responders. The State stands ready to offer any support."

- With wires

More to come