By Elisha Fieldstadt, Staff Writer, NBC News

t least three people were killed Saturday in a shooting at a shopping mall in Columbia, Md., near Baltimore, police said.

At a news conference, Howard County, Md., Chief of Police Bill McMahon said that the gunman was believed to be one of the dead and that the shooting took place in a single store on the upper level of The Mall in Columbia.

"We do not think at this point that there's any more shooters in or around the mall," McMahon said. He added that many people were still inside the mall sheltered inside stores — more than two hours after shots were fired.

Earlier, police tweeted live reports from the mall, confirming three deaths and then adding that one of the victims was located near a gun and ammunition. "One of the dead subjects is the suspected shooter," the department later tweeted.

Two other people with "minor injuries that were not directly related to the shooting" were transported to the nearby Howard County General Hospital, police said.

Police said the first 911 call reporting a shooting at the mall was received at 11:15 a.m.

McMahon said shoppers followed instructions from mall security to seek shelter.

"They have done what we have encouraged them to do in one of these situations," McMahon said, adding that he had not heard reports of stampedes. Police had staged drills will mall staff in the past to prepare for an emergency situation, he said.

Those still inside the mall were being evacuated to a movie theater and allowed to leave from there if they did not witness the shooting, Howard County Fire Dispatch calls indicated. The movie theater is separate from the mall, on the opposite side of a parking lot, according to Mall in Columbia maps.

Three witnesses told that the gun used in the shooting was a shotgun, though police have not yet confirmed that.

A man whose daughter was in the mall told that she told him over the phone that she was taking shelter inside of a Bank of America inside the mall with dozens of others.

Another witness told that she heard what she thought was "something being dropped," but then realized it was gunshots. "They just kept shooting and it didn't stop," she said.