The suspect was a 26-year-old temporary employee at the Rite Aid distribution center in northeastern Maryland, Harford County Sheriff Jeffrey Gahler told a news conference. She had been taken into custody in critical condition that morning. Her name was not immediately released.
It appears only one weapon, a handgun, was used and no shots were fired by responding law enforcement officers, Gahler said.
WATCH the noon news conference given at the scene
Krystal Watson, 33, said her husband, Eric, works at the facility and told her told her that the suspect had been arguing with somebody else near a time clock after a "Town Hall meeting."
"And she went off," she said.
Watson said her husband told her the shooting started in a break room.
"She didn't have a particular target. She was just shooting," Watson said as she drove away from a fire station where relatives tried to reunite with loved ones.
"She didn't aim. She just shot," Watson said.
Area hospitals reported receiving five patients from the incident.
Johns Hopkins Bayview Medical Center in Baltimore said it was treating four patients with gunshot wounds. Two were in stable condition and two who were seriously injured.
A spokesman for a health system that includes Christiana Hospital in Newark, Delaware, said one patient was being treated there. Christiana Care Health System spokesman Hiran Ratnayake said the person was in serious condition.
Gahler said the call about shots fired came in at about 9:06 a.m. and deputies and other officers were on the scene in just over five minutes.
Mike Carre, an employee of a furniture logistics operation next to the distribution center, said 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.
Carre said the man told him the shooter "just came in in a bad mood this morning. He said she's usually nice. But today, I guess it wasn't her day. She just came in to pick a fight with someone."
"She pulled out a gun and she just started shooting at her co-workers."
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 center.
Reggie Rodgriguez's mother works at the distribution center. His wife had tried calling her numerous times but got no answer.
"I was calling her all morning. It went to voicemail because they keep their phones in lockers," said Kelly Rodriguez, 40.
When they finally reached his mother Thursday afternoon, Reggie Rodriguez said, "That's all I wanted to do: Hear her voice." His mother was uninjured. He said she sounded relieved the crisis was over.
A law enforcement official said authorities were working their way through the distribution center to clear the facility.
"We obviously need to methodically go through the entire complex, ensure there are no other victims inside, ensure that there is no one that was hiding from violence and ensure that there were no other suspects," said Christie Hopkins is communications director for the sheriff's office.
"At this time, we are confident that the suspect acted alone. And that person is in custody. We do not believe there is any further threat to the community now, but we still have to completely clear that building."
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, described the warehouse 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."
The Maryland incident comes one day after multiple people were shot in incidents in Pennsylvania and Wisconsin.
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