Phil Davis, who covers courts and crime for the Capital Gazette, tweeted about the at the company's building in Maryland Thursday, saying "There is nothing more terrifying than hearing multiple people get shot while you're under your desk and then hear the gunman reload."
Five people were killed in the shooting, according to authorities. One suspect is in custody.
"Gunman shot through the glass door to the office and opened fire on multiple employees," Davis tweeted.
He later said he didn't tweet from under his desk and that he was safe when he started posting on social media.
Later, in an interview, Davis said it "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, The Baltimore Sun reported. "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 law enforcement source told CBS News the suspect in the shooting is a white male in his 20s and that he didn't have identification on him. Two law enforcement sources said the suspect used a shotgun.
The shooting prompted a massive police response, with employees evacuated from the building with hands raised above their heads.