President Donald Trump and Press Secretary Sarah Sanders also tweeted remarks following the massacre, being joined by numerous politicians throughout the country.
Thursday's shooting in Maryland led police forces in cities such as Los Angeles, New York City and Boston to have officers conduct patrols near major media outlets.
Police described the suspect - who was being questioned in custody - as a white adult male who was armed with a rifle or a shotgun and appeared to have acted alone.
Jarrod W. Ramos was captured after the shooting at the offices of Capital Gazette Communications, according to a senior law enforcement official speaking on condition of anonymity.
The official wasn't authorized to discuss the ongoing investigation publicly and spoke to the AP on condition of anonymity.
The fear journalists feel today is no different from the one high school students felt after Parkland or clubgoers felt after the Pulse or people at music festivals felt after Las Vegas. Hiassen was an assistant editor and columnist.
Krampf said the gunman was a Maryland resident, but didn't name him.
"A single shooter shot multiple people at my office, some of whom are dead", reporter Phil Davis tweeted on Thursday afternoon.
"There is nothing more terrifying than hearing multiple people get shot while you're under your desk and then hear the gunman reload", Davis said. It never is. Killing like this is nothing but madness, no matter what terms it's couched in.
Meanwhile, investigators said they would seek to learn more of the gunman's motives.
Threats against the paper had been made as recently as the day of the shooting, Anne Arundel County Deputy Police Chief Bill Krampf said.
In July 2012, Ramos filed suit against Capital Gazette for defamation, according to the 2015 court filing. The case was dismissed, and an appeals court upheld the dismissal.
Simon also remembered his friend McNamara, who he called "a careful, committed and lifelong journalist who first honed his craft as a sports reporter on the University of Maryland Diamondback, where I had the pleasure of working with him".
"Every shooting like this is horrific but one that occurs in a place of journalism is particularly disturbing and brings back the flood of memories of that tragic day for me", said Andy Parker, whose daughter Alison was one of the Roanoke victims. "Reporters shouldn't have to hide from gunfire while doing their jobs", she said in a statement.
"I heard footsteps a couple of times ... I was breathing really loud and was trying not to, but I couldn't be quiet", she added.
Journalists at the historic Maryland newspaper took to social media to seek help and inform others what was happening during a gunman's deadly attack.
"It was quite obvious that this person had some sort of vendetta against the Capital newspaper", said Lt. Ryan Frashure, a county police spokesman. "They don't make a lot of money". "It's immoral that their lives were at risk".
"Prior to departing Wisconsin, I was briefed on the shooting at Capital Gazette in Annapolis, Maryland".
In a later interview with the Gazette's parent company, the Baltimore Sun, Davis reflected further on what he had been through. They will try to recover any of Ramos' electronic devices and see if he has any social media accounts.
A US official told the Associated Press that the suspect damaged his fingers in what is believed to be an effort prevent investigators from quickly identifying him through his fingerprints.
Two law enforcement sources said his fingerprints appeared to have been altered, which made it hard to identify him, but one law enforcement source said he was identified through facial recognition software.
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