Anne Arundel County Executive Steve Schuh said the shooter is in custody.

"Gunman shot through glass door and opened fire. Employees could hear gun being reloaded," Xinhua quoted a reporter for the newspaper, Phil Davis, as saying.

"There is nothing more terrifying than hearing multiple people get shot while you're under your desk and then hear the gunman reload," he tweeted.

Police are still working to search the Capital-Gazette building, according to the Anne Arundel Police, urging public to avoid area via Twitter.

US President Donald Trump has been briefed on the shooting, said White House deputy press secretary Lindsay Walters.

Police cars have swarmed the scene and people were seen streaming out of the building with their hands up.