The attack took place at a skateboard shop called Zumiez on the upper level of the Mall in Columbia, a suburb of both Baltimore and Washington, according to Howard County Police Chief William J. McMahon.
McMahon said authorities ran into difficulty in identifying the gunman because of concerns that he might be carrying explosives.
The two victims were a man and a woman, both in their 20s. They were employees of the shop where the attack occurred. Their names were not released.
Five other people were injured, none seriously.