The shooting happened on Thursday morning at a pharmacy distribution centre in Perryman, near Baltimore.
The attacker, Snochia Moseley, 26, died from a self-inflicted gunshot wound. The motive for her attack is unclear.
Mass shootings by women are rare, with more than 95% of such attacks in the US carried out by men.
‘Officers fired no shots’The shooter was a temporary employee who reported for work as usual that morning, Harford County Sheriff’s Office said.
But shortly after 09:00 local time (13:00 GMT) she began shooting outside, then inside the facility, with a 9mm Glock handgun that she had brought with her.
Police were on the scene within five minutes, by which time the shooter had killed three people and injured three more.
Officers found the suspect in a critical condition, with a self-inflicted gunshot wound to the head – she died later in hospital.
She was from Baltimore County, police said.
She owned and had registered the handgun, and had taken several magazines of ammunition with her, police said.
The sheriff’s office initially urged residents to avoid the area, which is about 40km (25 miles) north-east of Baltimore.
But it later tweeted that “we do not believe there is any further threat to the community”.