Police confirmed on Friday morning (Australian time) that they were responding to an "active shooter", at the offices of the Capital Gazette newspaper.
Police said an "explosive device" was also found at the scene.
Phil Davis, a Capital Gazette reporter who was in the building at the time of the shooting, said multiple people were shot, as others hid under their desks.
"Gunman shot through the glass door to the office and opened fire on multiple employees. Can't say much more and don't want to declare anyone dead, but it's bad," Davis wrote on Twitter.
"There is nothing more terrifying than hearing multiple people get shot while you're under your desk and then hear the gunman reload," he continued.
"I'm a police reporter. I write about this stuff - not necessarily to this extent, but shootings and death -- all the time," Davis later told the Baltimore Sun.
"But as much as I'm going to try to articulate how traumatizing it is to be hiding under your desk, you don't know until you're there and you feel helpless."
According to CNN, a white male has been taken into custody but no motives are known.
President Trump has responded to the shooting, writing on Twitter: "My thoughts and prayers are with the victims and their families. Thank you to all of the First Responders who are currently on the scene."