The shooting happened on Thursday at a pharmacy distribution centre in Perryman near Baltimore BBC reported.
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 per cent of such attacks in the US carried out by men.
The shooter was a temporary employee who reported for work as usual that morning Harford County Sheriff s Office said.
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 persons 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 40 km north-east of Baltimore.
But it later tweeted that "we do not believe there is any further threat to the community".
--IANS pgh/
