US police have confirmed that three people have been killed in a shooting at a mall outside the city of Baltimore, Maryland.

The shopping centre is just 48 kilometres north of the capital, Washington DC. The identities of those killed are unknown. But police believe one of the dead was the shooter. A gun and ammunition was found near the body.

A 911 call at about 11:15 am local time reported shots being fired at the mall in Columbia, Howard County. Police say they are allowing shoppers to leave after determining there is no longer a threat.