WASHINGTON (AFP) - A gunman who opened fire in a shopping mall near Washington, killing two people before taking his own life, was a 19-year-old armed with a shotgun and crude homemade explosives, the police said on Sunday.

Dario Marcus Aguilar fired six to eight shots during the attack at the Columbia Mall Saturday, killing two employees of a skateboard shop, Howard Country police chief Bill McMahon said.

The police are investigating whether the shooter, who was from the nearby town of College Park, Maryland, had any prior relationship with the victims.

"Once we can establish if there is any relationship or not, that will certainly help us to identify the next big question which is why this happened," Mr McMahon told reporters.

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

Five injured shooting victims were also treated and released, Howard County General Hospital said on Saturday. One had suffered a gunshot wound to a foot.

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