Staff at the Capital Gazette newspaper in Annapolis said he shot through a glass door into the newsroom.
"There is nothing more terrifying than hearing multiple people get shot while you're under your desk and then hear the gunman reload," tweeted reporter Phil Davis.
Police said a suspect was now in custody and was being questioned.
"We've had a terrible shooting incident here this afternoon," county executive Steve Schuh said in a news conference.
"There are several people who have died from this incident."