Authorities responded to 911 reports of an "active shooter" and found three people dead, including one person who was found near a shotgun and ammunition.
Detectives said the gunman died from a self-inflicted gunshot wound.
The suspected shooter had been tentatively identified, but police have not yet released his name. According to ABC News, all three of the dead were found in the Zumiez store, a skateboarding shop, on the upper level of the mall.
Howard County police identified the two believed to have been shot by the gunman as Brianna Benlolo, 21, of College Park, Md., and Tyler Johnson, 25, of Ellicott City, Md. Both were employees of the skateboard store.
Police said in a news release Saturday night they found and disabled "two crude devices that appeared to be an attempt at making explosives using fireworks." Police continued to search the mall overnight. The mall will stay closed Sunday.
The mall was put on lockdown as police asked people to stay inside the shopping center until the area was clear.
Laura McKinzles of Columbia works at a kiosk in the mall. She said she heard between eight and 10 gunshots, followed by people running and screaming. She ran into the backroom of a perfume store and locked the door.
A motive for the shooting was under investigation.
"We don't know a motive yet," Howard County Police Chief William J. McMahon said. "We are very confident that it was a single shooter, and there was not another shooter in the mall."
According to ABC News, law enforcement officials are considering the possibility that the incident was a domestic dispute involving a man, his estranged girlfriend, and her boyfriend.
Columbia is a suburb of both Baltimore and Washington.
ABC News and The Associated Press contributed to this report.