Incident comes a day after separate shootings at a Wisconsin workplace and outside a Pennsylvania court.

A woman killed three people in the United States at a warehouse in northeast Maryland before taking her own life, police said on Thursday.

Harford County Sheriff Jeffrey Gahler told reporters on Thursday that police responded to the shooting at about 9:06 local time (13:06 GMT) at a Rite Aid distribution centre in the town of Aberdeen, which is about 56km north of Baltimore.

Gahler said there was no longer a threat at the site and an investigation was under way.

The sheriff’s office identified the suspected assailant as a 26-year-old woman. Gahler said investigators have not yet determined a motive, but a law enforcement source told the Reuters news agency the shooting was believed to be have been sparked by a work-related grievance.

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

Maryland state police block the road that connects the industrial business park, where several people had been shot, in Harford County [Jose Luis Magana/AP Photo]

Thursday’s shooting comes less than 24 hours after police said three individuals were wounded when a gunman opened fire outside a crowded courtroom in southwestern Pennsylvania. The suspect, Patrick Dowdell, was shot and killed by police.

Also on Wednesday, at least four people were wounded when a software worker shot his fellow employees at a business near Madison, Wisconsin, officials said. The suspect, who police identified as 43-year-old Anthony Tong, died after being shot by police responding to the incident. Three people remain in serious condition.