At least five people died and several others were injured in the shooting, according to officials.
Authorities in several USA cities, including NY, responded to the violence in Maryland by sending police to offices of media outlets.
The suspected gunman was not cooperating with authorities and had no identification, officials said. Smoke bombs and flash bang devices were found in his backpack, a source said.
"That person is at criminal investigations unit down in Crownsville [Md.]", Frashure said.
Police went to the offices of the Baltimore Sun as a precaution, that paper reported.
Police, who were on the scene between 60 and 90 seconds after receiving the report, say that the building has been evacuated, and officers are continuing to search.
Annapolis police lieutenant Timothy Seipp told reporters that a suspect was in custody, but authorities were searching the area for other potential suspects.
He credited active shooter training for the quick response. He said he hid under his desk and employees heard the gunman reload.
"Absolutely devastated to learn of this tragedy in Annapolis", Gov. Larry Hogan said.
Sources tell CBS News five other people were injured.
The wounded are being treated at two local medical centers, including Anne Arundel Medical Center. The seriousness of the patient's injuries wasn't immediately available.
Local news reports showed people walking out of the building with their hands up and being escorted by police through the auto park outside the building.
"I don't know why".
"A single shooter shot multiple people at my office, some of whom are dead", he tweeted. It is unclear whether the facial recognition was from a photograph or witness, the source said.
President Donald Trump tweeted: "Prior to departing Wisconsin, I was briefed on the shooting at Capital Gazette in Annapolis, Maryland".