CNN:

Gunfire erupted in a Columbia, Maryland, shopping mall Saturday morning, leaving at least three people dead, according to the Howard County Police Department's Twitter account.
Authorities reported that an active shooter was on the second floor of the sprawling mall in Columbia in Howard County, according to CNN affiliate WBAL. Local scanner reports had authorities requesting multiple mass casualty units.

Images on Twitter reportedly from the scene showed mall employees and customers hiding in a stock room. The mall was on lockdown. Another Twitter image reportedly from the scene showed a bullet-riddled wall.

Update: Police are confirming that one of the dead is the shooter.