A woman has killed three of her coworkers and wounded several others at a business in Maryland, in the United States, before shooting herself twice, officials have confirmed.

Another three were wounded, but were expected to survive, Harford County Sheriff Jeffrey Gahler said on Friday (Thursday local time).

The shooter has been identified as 26-year-old woman Snochia Moseley, a part-time employee of the Rite Aid pharmacy warehouse in the town of Aberdeen, about 50 kilometres north-east of Baltimore.

Ms Moseley had been agitated in the weeks prior to her death, Mr Gahler said.

“Frankly, when someone does something like this, such violence against other human beings, we’re never going to make sense of it or understand it fully,” Mr Gahler said.

FBI tactical agents prepare as police search for the shooter who fled the scene.

After arriving at work on Friday morning, Ms Moseley killed three of her colleagues before shooting herself in the head in an apparent suicide attempt and when unsuccessful, shot herself again.

She was taken into custody and to hospital in a critical condition, but later died from the self-inflicted wounds.

The three victims have been identified as two 41-year-old women from Baltimore County and a 45-year-old man from Baltimore County.

A car belonging to one of the victims at the crime scene. Photo: AAP

Ms Moseley had only worked at the distribution centre for two weeks before the shooting. It is unclear what motivated the attack but authorities said it appeared the shooter targeted her victims.

The incident is the third mass shooting in the US within the past 24 hours, after a shooting at an office complex in Wisconsin that wounded three people on Wednesday, while a gunman wounded four people in Pennsylvania later in the day. Police killed both shooters in those incidents.

It appears there was only one weapon, a handgun, that was used and there were no shots fired by responding law enforcement officers, Sheriff Gahler said. The woman was also believed to have acted alone.

Sheriff Gahler also said “multiple” people had been killed and wounded in the shooting, without giving any other details on casualties.

He said the call about shots fired came in from the Rite Aid distribution centre at about 9.06am on Thursday (local time).

The attack comes almost three months after a man armed with a shotgun attacked a newspaper office in Annapolis, Maryland, killing five people.

Authorities accused Jarrod W Ramos of attacking The Capital Gazette because of a longstanding grudge against the paper.

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

No law enforcement officers fired shots during their response, he said.

Johns Hopkins Bayview Medical Centre received four patients with “priority one gunshot wounds,” said spokeswoman Kristin Mears.

She declined to provide further details, including the victims’ conditions, until their families were notified.

-with AAP