Police are searching for clues and evidence after a "targeted attack" left five people dead and turned journalists in an Anapolis, Maryland newsroom into the latest victims of a mass shooting in America.

The scene outside of the newspapers The Capital and Maryland Gazette continued to be swarmed by investigators overnight as they tried to determine why a man they said was wielding a shotgun would pack smoke bombs to confuse the scene in the building's lobby before breaking windows and taking lives with gunfire as he walked between desks in the office.

At the same time, the very journalists whose newsroom had been attacked worked through the evening to ensure that the community it serves would open their front doors in the morning to find the newspaper there waiting for them.

"This was a targeted attack on the Capital Gazette," William Krampf, the Anne Arundel County acting police chief, said Thursday evening during a press conference, referring ot the joint name that appears at the office to describe the two papers that work inside. "This person was prepared today to come in. This person was prepared to shoot people. His intent was to cause harm, and as I've stated before, the investigative part of this is going to be thorough, and it's going to take some time,"

Four of the victims died at the scene in the office, with one dying later in hospital, Anne Arundel County Executive Steve Schuh said.

Shortly after the shooting, police rushed to the scene at the offices of local newspaper The Capital within a minute of being called and engaged with the suspect. The alleged shooter, identified as a white adult male in his late 30s, was found under a desk with a weapon nearby and taken into custody for questioning.

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

Phil Davis, a crime reporter at the The Capital - which also publishes sister publication the Maryland Gazette - said that multiple people had been shot in the newsroom.

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Mr Davis tweeted that the "gunman shot through the glass door to the office and opened fire on multiple employees."

“There is nothing more terrifying than hearing multiple people get shot while you’re under your desk and then hear the gunman reload,” Mr Davis added.

In a follow-up interview, Mr Davis told The Baltimore Sun that even his extensive experience with violence as a part of his job could not prepare him for the shooting, which he said left the newsroom looking "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," Mr Davis told that paper, which owns The Capital, where he works. "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".

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Police said the weapon used was a "long gun". Lt. Ryan Frashure, of the Anne Arundel County police department said that officials “anticipate this being a very long investigation”.

"At this point we’re still talking to the individual [suspect] and trying to figure out the motive,” Mr Frashure added.

Local news reports showed people walking out of the building with their hands up, police said 170 people were evacuated from the building.

A relocation checkpoint was established for those separated during the incident at the nearby Lord & Taylor’s in the Annapolis Mall.

The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) Baltimore said they were responding to the incident.

President Donald Trump tweeted about the incident from aboard Air Force One, returning to Washington from an event in Wisconsin.

"My thoughts and prayers are with the victims and their families," he wrote. "Thank you to all of the First Responders who are currently on the scene."

Prior to departing Wisconsin, I was briefed on the shooting at Capital Gazette in Annapolis, Maryland. My thoughts and prayers are with the victims and their families. Thank you to all of the First Responders who are currently on the scene. — Donald J. Trump (@realDonaldTrump) June 28, 2018

Maryland Governor Larry Hogan said on Twitter: “Absolutely devastated to learn of this tragedy in Annapolis.”

Mr Hogan said he was in contact with County Executive Steve Schuh, and that Maryland State Police were on the scene assisting county police.

At a later news conference, Mr Hogan called it a "tragic situation" but a number of "brave people came in and made sure it was not any worse".

“Please, heed all warnings and stay away from the area. Praying for those at the scene and for our community,” he wrote.

Senator Chris Van Hollen wrote on Twitter, “My heart is with the families, friends, and loved ones of the victims as we learn more about this terrible situation. We must unite to end the violence.”

Police were also sent The Baltimore Sun newsroom in Baltimore, with the Sun owning the Capital Gazette. Police said that the move was a precaution and that there was no threat. The New York Police Department said that similar protection was being sent to major newsrooms in the city as a precaution against potential copy-cat incidents.