But some stores continued to be a draw. Outside the Apple store, a small crowd waited for the opening as employees huddled in the back. A cheer went up when the shoppers were allowed in.
The governor visited the mall Monday, a decision he said he hoped would symbolize how all the people of the state are standing behind Columbia.
"The tragedies that unfolded here over the weekend reminded us that in places all around our state, that all of us are human, therefore all of us are vulnerable, " O'Malley said. "But also there are a lot of good people in our state who understand the senselessness and tragedy of violence."
Ashley Venable, the mall's senior general manager, thanked the Police Department for its support.
"As a community we will heal together," she said.
McMahon said his officers are working to uncover a motive in the shooting. He said he believes workers and shoppers will feel more at ease if it turns out that the shooter and victims knew one another. Random attacks can be more difficult to understand than those explained in the context of some existing relationship, he said.
"In some ways, if this was a domestic-related incident, or if we know there was some personal conflict between the shooter and the two young people who were killed, we would be able to say, 'Yes it's a tragedy — it is — but I am not involved in that, so I am safe,'" McMahon said. "Until we can establish that, the prospect of this just being somebody that came in with no known relation to the mall or the victims, to come in and shoot people, is very troubling."
At the security briefing, mall employees packed into the AMC movie theater, the last to arrive lining up on the stairs to listen. McMahon urged staff to try to return to their normal routines.
Employees, many of whom raised their hands when asked if they'd been at work Saturday, had a number of questions for McMahon and Ulman, who also attended the briefing.
One employee asked what advice police have for helping them spot gunmen in the future.
McMahon said it can be difficult to spot a gun when people are bundled up against the cold, but he encouraged staff to call police whenever something looks amiss.
Another asked whether the mall needed more security cameras, after McMahon said there were some holes in surveillance footage of the shooter's movements. He said officials would examine the security system.
Another asked whether it's right for a 19-year-old to be able to buy a shotgun legally. Police have said Aguilar used a Mossberg 12-gauge shotgun purchased last month in Montgomery County.
Ulman and McMahon said there would be time to explore such issues once police had a more detailed picture of what happened Saturday.
After the briefing, employees gathered in the mall, many pausing at the bottom of an escalator to toss white flowers with long stems into the pool that will serve as a memorial to Saturday's victims.
Some lined up to write messages in a book of condolences, while across the way two people raised the grate on the Gymboree store to get ready to open for business.
Some said it was too soon to tell how shoppers would react, and if they would return. David Youngblood, 47, said he was eager to get back on the job selling Traeger Grills. Youngblood was there Saturday, working on his iPad, when the shooting happened. "I literally dropped my iPad and ran off," he said.
"Either they're going to shy away or maybe they're going to do like Boston Strong … Columbia Strong," he said, referring to the public response following the Boston Marathon bombing. "I'll find a way to sell grills whatever."
Later in the afternoon, Youngblood said business had been slow. Winter is not a great time for selling grills anyway, he said. But the weekend before the Super Bowl might have been a bright spot.
By early evening, he said he had yet to make a sale.
Baltimore Sun Media Group reporter Luke Lavoie contributed to this article.