ANNAPOLIS, Md. (CBS NEWS/CBSLA) — Four people were killed Thursday in a shooting at a local newspaper building in Maryland, a senior federal law enforcement source tells CBS News. One suspect is in custody. The shooting at the Capital Gazette newspaper prompted a massive police response, with employees evacuated from the building with hands raised above their heads. The Gazette is a local newspaper owned by The Baltimore Sun. The first shots were reported at 2:40 p.m. after the shooter entered the building, which also houses other businesses, CBS Baltimore reports. Phil Davis, a Gazette reporter, tweeted that he was in the building when the shooting happened. Davis said via Twitter the gunman shot through the glass door to the office and opened fire on multiple employees. He said he hid under his desk and employees heard the gunman reload. There is nothing more terrifying than hearing multiple people get shot while you're under your desk and then hear the gunman reload — Phil Davis (@PhilDavis_CG) June 28, 2018 Massive police response to shooting in my newsroom in Annapolis. @capgaznews pic.twitter.com/M1Bjwa0mMh — Joshua McKerrow (@joshuamckerrow) June 28, 2018 “We’ve had a terrible shooting incident here this afternoon,” Anne Arundel County Executive Steve Schuh said during a press conference. Schuh confirmed several died. Several others who were injured are being treated at local hospitals. Officials said the suspect was being interrogated. Governor Larry Hogan called it a “tragic situation.” He and Schuh praised local law enforcement for their quick response, saying they responded within 60 seconds. Sources tell CBS News five people were injured. One patient is being treated at the University of Maryland Medical Center, a spokesperson confirmed. The seriousness of the patient’s injuries wasn’t immediately available. Two other local hospitals said they had not received patients. Two law enforcement sources tell CBS News the suspect used a shotgun. One law enforcement source tells CBS News the suspect is a white male in his 20s and didn’t have identification on him. The suspect is refusing to identify himself, CBS News confirms. Police respond to a shooting in Annapolis, Maryland, June 28, 2018. (Photo credit: SAUL LOEB/AFP/Getty Images) Lt. Ryan Frashure of the Anne Arundel County Police confirmed there were “several injuries” as a result of the shooting but declined to provide further details. Frashure said officers are still clearing the building and must look for other dangers, such as bombs and other shooters. He urged the community to avoid the area and advised that a re-unification center had been set up at a nearby mall. White House spokeswoman Lindsay Walters says President Trump has been briefed on the shooting and “our thoughts and prayers are with all that are affected.” Officials said the Bureau of Alcohol, Tobacco, Firearms and Explosives were responding to the scene. Capital Gazette Communications’ staff includes three people on the executive staff as well as seven editors, according to CapitalGazette.com. Eight staff writers cover topics including courts, entertainment and education. Security was stepped up at prominent media outlets in Los Angeles, New York City and other potential targets following the shooting. #NYPD at media organizations in & around #NYC (pictured here at CBS broadcast center) out of abundance of caution. @CBSNewYork pic.twitter.com/40lpbfHwQF — Alice Gainer (@GainerTV) June 28, 2018 Police in Los Angeles moved to increase patrols outside various news outlets across the city, including the L.A. Times. Deputy Chief Robert Arcos told The Times the added patrols were considered a precaution and “reassurance,” and said there were no specific threats. (This is a developing story. © 2018 CBS Interactive Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.