Three people were killed on Thursday morning in a shooting at a Rite Aid distribution centre in northeast Maryland, officials said.

A law enforcement official with knowledge of the shooting stressed that the number of dead is based on preliminary information. The official wasn't authorised to discuss details by name and spoke on condition of anonymity to The Associated Press.

A suspect was taken into custody and was in critical condition, Harford County Sheriff Jeffrey Gahler told a news conference.

It appears only one weapon, a handgun, was used and no shots were fired by responding law enforcement officers, Gahler said. He also said multiple people had been killed and wounded in the shooting, without giving any other details on casualties.

A Baltimore hospital said it had received four patients with gunshot wounds from the shooting.

Johns Hopkins Bayview Medical Center spokeswoman Monica Stone said in an email on Thursday that she was unable to provide details about the patients' conditions.

Gahler said the call about shots fired came in from the Rite Aid distribution center at about 9:06am and deputies and other officers were on the scene in just over five minutes.

“We are so preliminary in this investigation,” Gahler said. “It's so important that we deal in facts.” He said authorities don't want to “make it worse” for the families involved by giving out incorrect information.

Mike Carre, an employee of a furniture logistics operation next to the distribution center where the violence erupted, says he helped tend to a wounded man.

Carre locked the doors of his workplace after the injured man came hobbling in, bleeding from his leg. He called 911 from a bathroom before helping colleagues wrap the man's blood-soaked jeans above his injury to cut off blood flow.

At a nearby fire station, family members were waiting to be reunited with loved ones. Police blocked off the road outside but were waving in cars driven by people who said they were there to meet up with people who were at the distribution centre.

The attack came nearly three months after a man armed with a shotgun attacked a newspaper office in Annapolis, Maryland, killing five staff members. Authorities accused Jarrod W Ramos of attacking The Capital Gazette because of a longstanding grudge against the paper.

The FBI described the Aberdeen incident as an “active shooter situation” and said its Baltimore field office was assisting.

In a tweet, Maryland Gov Larry Hogan said his office is monitoring the situation in Aberdeen and that the state stands ready to offer any support.

Susan Henderson, spokesman for the drugstore chain Rite Aid, said the shooting happened on the campus of a company distribution center in Aberdeen. She described it as a support facility adjacent to a larger building.

Harford County Executive Barry Glassman said that unfortunately, incidents like this are “becoming a too-often occurrence not only in Harford County but in the country.”