At least five were killed and several others "gravely injured" during a shooting at the offices of the Capital Gazette newspaper in Annapolis, Maryland, law enforcement officials said Thursday.

Anne Arundel County Executive Steve Schuh told reporters at a briefing following the shooting that a male suspect was in custody and was being interrogated. Authorities said they had not yet determined a motive, and declined to provide details about the weapon that was used beyond saying that it was a "long gun."

Law enforcement also identified what they believed to be an explosive device on the scene, an official said. More than 170 people were evacuated from the building after the shooting, he said.

The suspect in Thursday's shooting at a Maryland newspaper was identified as Jarrod Ramos, born Dec. 21, 1979, according to three senior law enforcement officials.

A crime reporter at the Gazette provided harrowing details about the incident on his Twitter account in the moments after it occurred.

“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,” crime reporter Phil Davis wrote in one post. “There is nothing more terrifying than hearing multiple people get shot while you’re under your desk and then hear the gunman reload.”

Law enforcement responded to the incident in about 60 seconds, Gov. Larry Hogan told reporters following the shooting. More than 10 agencies were assisting in the investigation, a law enforcement official said.

"We don't have all the information yet, and we can't give all the information yet, because this is an active crime scene and investigation," Hogan said.