Three people died and five others were injured Saturday in a shooting at a popular shopping mall outside the US capital, amid signs the suspect may have intended to kill more.

Howard County Police Chief Bill McMahon said the as yet unidentified alleged gunman had "a large amount of ammunition" still on and around him when officers found his corpse in a store alongside the two victims at the Columbia Mall in Maryland.

"Because of concerns about any other weapons he may have or explosives, we are approaching this with an abundance of caution," McMahon said, stressing that an investigation had just begun.

"We're getting assistance from some of our federal partners and making sure there are no explosives on the body of the deceased."

First responders were alerted to the mid-morning gunfire at the mall, a favorite weekend spot for young families about 45 minutes outside downtown Washington, by an emergency police call indicating shots were fired inside the two-story shopping center.

"#HoCoPolice responded to 911 call in less than 2 minutes. Found victims and suspect dead from gunshots upon arrival," the Howard County Police Department tweeted.

It said officers would be posted "throughout the night" both inside and outside the mall, which will remain closed for the day.

Police identified the victims as Brianna Benlolo, 21, of College Park, Maryland, and Tyler Johnson, 25, of Ellicott City, Maryland. Both were employees of skateboard store Zumiez.

The identity of the shooter is still unknown, and police have yet to identify a motive.

Howard County General Hospital said all five injured shooting victims have been treated and released. One had suffered a gunshot wound to foot.

"The other four were either medical conditions or twisted ankles, things like that as people have were moving away from a very, very chaotic and dangerous situation," McMahon said.

The mall of about 200 stores, which has an indoor carousel and play area, is a favored weekend destination for young and old alike.

Many people living in Columbia, a planned community replete with parks and good schools, work in either Washington or Baltimore.

The Baltimore Sun newspaper said shoppers reported hearing gunfire before fleeing the upscale mall, home to several department stores such as Lord & Taylor, Macy's and Nordstrom.

NBC television spoke to one man who said he was in phone contact with his daughter, who was taking shelter in a Bank of America branch inside the shopping center, along with dozens of others.

The young woman described a chaotic scene as shoppers were told to evacuate or quickly take shelter.

"People were panicking," her father said.

An employee at one of the stores described the mayhem as shots resonated through the mall.

"In today's world that we live in, when you hear gunshots, you run. There's nothing else to do," he told CNN.

The shooting is the latest in a slew of gun-related rampages in the United States.

The most notorious in recent history took place in Newtown, Connecticut, in December 2012, when a young gunman entered an elementary school and opened fire. In 10 minutes, the 20-year-old shot and killed 26 people before taking his own life.

The Newtown attack briefly reignited the US gun control debate, triggered every time there is a major shooting, although attempts to pass tougher laws have made little headway in the US Congress.