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Howard County Police said they believed the mall in Columbia, Maryland, was secure and they were contacting shoppers to help them leave the complex.
Police said they did not immediately know the identities of the dead and did not know the motive of the shooter. The shooting happened at a skate shop called Zumiez on the mall's upper level.
An emergency 911 call was received at about 11:15am reporting that shots had been fired at the Mall of Columbia, a sprawling shopping center with about 200 stores about 48 km north of Washington, D.C.
"We had officers quickly get into the area and we were able to identify three victims at an upper level store in the Columbia Mall, one of those victims appears to be the shooter," Howard County Police Chief Bill McMahon told reporters.
"We are very confident that it was a single shooter and there are no other shooters in the mall," he said.
Five people were injured during the shooting, but only one suffered a gunshot wound, according to a spokeswoman at Howard County General Hospital, where the victims were taken for treatment. All the victims were in good condition, spokeswoman Susan Case said. Police said one of the victims had a gunshot wound to the foot.
The suspected shooter was found near a shotgun and ammunition, police said.
At the time of the shooting the mall was crowded with weekend shoppers, many of whom sheltered in place after hearing the shots or seeing people fleeing.
"So right now we still have many, many people still in the mall who have secured themselves inside stores," McMahon said, adding that officers were helping people leave the building.
Colin Reddy, who works at the mall, told CNN he heard about eight or nine shots in all.
"We thought it was construction because there's a lot of construction going on at the mall right now. Then I heard it again. Like 'boom, boom, boom'. And then everybody started running," he said.
Reddy said he and the other employees closed the store's gate and waited in a back room for police to arrive.
Tonya Broughton of Silver Spring, Maryland, said she and a friend were getting facials when they noticed other shoppers running.
"It was panic. We just headed for the nearest store, Victoria's Secret," she said, adding that the store's employees herded everyone to the rear of the store.
"They were very, nice, keeping us calm. They told is we were safe," she said.
The attack in Columbia follows a shooting at a New Jersey mall in November in which a gunman fired at least six shots without hitting anyone, sparking a mass evacuation of the complex, then killing himself.