Five people were killed Thursday in a shooting at the Capital Gazette building in Annapolis, Maryland, said acting Anne Arundel County police chief Bill Krampf.
Multiple others were injured, Anne Arundel County Executive Steven Schuh said.
A reporter at the paper said a gunman fired through newsroom's glass door.
One person is in custody who is believed to be a shooter at the Gazette building, a law enforcement official told CNN.
The suspect initially refused to cooperate, a law enforcement source said. The suspect had no identification when he was apprehended and FBI found no information immediately in the system, the source said.
A law enforcement source said a shotgun was used in the shooting.
Anne Arundel County police Lt. Ryan Frashure said law enforcement has not confirmed where in the building the shooting occurred. He said the Capital Gazette is inside the building along with other businesses and doctors' offices.
'Like a war zone'
Phil Davis, a Capital Gazette police reporter, tweeted the "gunman shot through the glass door to the office and opened fire on multiple employees."
"There is nothing more terrifying than hearing multiple people get shot while you're under your desk and then hear the gunman reload," he said.
In an interview with the Baltimore Sun, Davis said the scene "was like a war zone".
"I'm a police reporter. I write about this stuff -- not necessarily to this extent, but shootings and death -- all the time," he said. "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."
'A newspaper we live with every day'
Susan O'Brien, a spokesperson for the city of Annapolis, said the publication "is a newspaper we live with every day."
"Our hearts are with the family," she told CNN.
O'Brien said Maryland Gov. Larry Hogan was on the scene.
Earlier, Hogan tweeted he was "absolutely devastated to learn of this tragedy in Annapolis."
"Devastating. My heart is with Capital Gazette and the people of Annapolis right now," Maryland Senator Ben Cardin tweeted.
The ATF and FBI said they are also responding to a shooting at that location.
Capital Gazette Communications publishes multiple newspapers, including the Capital and the Maryland Gazette.
The Capital is one of America's oldest newspapers, beginning life as the Evening Capital in 1884.
The company that publishes it started in 1727 with the Maryland Gazette. In 1767, Anne Catharine Green became the first female newspaper publisher in the country and the Maryland Gazette fought the stamp tax that started the American Revolution.
For many decades, there were two papers that served the region -- The Maryland Gazette, a weekly, and the Evening Capital, a daily.
In 1981, The Evening Capital became The Capital. And in 1994, the company that owned it launched one of the first newspaper websites in the US with CapitalOnline.com.
Today, The Capital serves Annapolis, Anne Arunedel County and Kent Island. It has a daily readership of about 67,000 and a Sunday readership of 83,000, according to the Baltimore Sun Group, which owns it.