The prime suspect of a newsroom shooting which killed five journalists used a legally purchased pump action shotgun.
Police in Annapolis said Jarrod Ramos used a gun he had purchased legally about a year ago to carry out the shooting in the Capital Gazette newsroom on Thursday.
Police Chief Timothy Altomare confirmed there had been a previous incident between the newspaper and the suspect, when he made threatening comments online to members of the newsroom in 2013.
However, the decision was made by the staff not to press criminal charges as they were concerned it would exacerbate the situation.
Ramos has now been charged with five counts of murder after the shooting. He has appeared in court, and been jailed pending a hearing date.
PC Altomare said the suspect had been "uncooperative".
In a press conference in Annapolis, PC Altomare said: "Maryland Police helped serve a warrant and we found evidence at the address to show the planning [of the attack].
"We have a bad guy and for his own reasons, he chose to do what he did."
He added there was evidence to show he intended to "kill as many people as he could".
Addressing whether police should have pressed charges despite the newspaper's decision not to, PC Altomare explained the investigator made a call based on evidence he had at the time.
He added: "Everyday we speak with someone who doesn't want to press charges. For a felony, we would go ahead, but on a misdemeanour, we would not always."
The suspect was caught using facial recognition software which has been controversial in Maryland.
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PC Altomare said without the technology, it would have been a much longer process for officers to track and arrest Ramos.