Police said officers were at the Capital Gazette building in Annapolis within 60 seconds of the shooting being reported at 2.40pm.
The shooter has been arrested and is being interviewed by police officers.
An official later described him as a white male who is believed to have used a shotgun or long gun in the attack.
He is not cooperating with investigators, the official added.
No motive has been disclosed but a law enforcement official said the shooting was being treated as a local incident, rather than terrorism.
The Gazette’s crime reporter Phil Davis wrote on Twitter that the gunman had 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.
He said that the shooter had hit multiple people at my office, some of whom are dead.
There is nothing more terrifying than hearing multiple people get shot while you’re under your desk and then hear the gunman reload, he added.
Mr Davis said he was tweeting after leaving the office and while waiting to be interviewed by police.
Meanwhile, a suspected explosive device has been found by police, who searched the newspaper office after the shooter was arrested.
Spokesman Lt Ryan Frashure said it had been taken care of and police do not believe there are any others at the site.
Anne Arundel Police Department’s acting chief William Krampf said It’s going to be quite a while determining what occurred, why it occurred and how it occurred.
We do have fatalities and we do have serious injuries.
We also have many, many witnesses being interviewed.
Police later estimated that three people were injured during the shooting.
Meanwhile, in New York, police said We are monitoring the reports from Annapolis and have deployed counter-terrorism teams to media organisations around New York City.
These deployments are not based on specific threat information, but rather a standard practice out of an abundance of caution.
Maryland Governor Larry Hogan said he was absolutely devastated to learn of this tragedy in Annapolis.
Congressman John Sarbanes, who represents the area, told Sky News that the newspaper had been part of the community for decades and decades.
They really feel the pulse of this community and now the community is going to feel the ripple effect from the newspaper itself.
US president Donald Trump tweeted, offering thoughts and prayers (for the) victims and their families.
News Source SkyNews