Residents were warned to "stay in their homes with doors locked at this time for their safety" by the police.
"This is the worst moment for any chief of police in any police agency, to have to deliver this news", police Chief Leanne Fitch said at an afternoon news conference.
Numerous emergency response personnel responded to the shooting Friday morning and remain at the scene.
A shooting rampage last month in that city left two people dead.
Tim Morehouse, another resident of the complex, said he heard someone shout: "Shut up!"
Cops added: "At this time, we can confirm multiple fatalities".
On Thursday, Ontario pledged more money for police and to keep suspects behind bars while they await trial on gun crime charges. "We were very close to one another, we spent all of our free time together". He said that police have been searching the complex floor-by-floor.More news: The media learned that note Putin gave Trump in Helsinki
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Const. Sara Burns, 43, is shown in this undated police handout photo.
Meanswhile, a large group of Fredericton police officers gathered outside the Chalmers hospital, and appeared to be consoling each other. "(Policing) is something that I think challenges you physically, mentally and socially; you need to be aware of those components.
Prime Minister Justin Trudeau offered his condolences via Twitter, and said that the government is "following the situation closely".
A 28-hour manhunt ensued before police caught up to the suspect and arrested him, in what was described as the second-deadliest attack on the Royal Canadian Mounted Police (RCMP) since four officers were ambushed on a Mayerthorpe, Alberta farm in 2005.
McLean said she and Costello had been living together for five years.
"We were looking forward to building years with just the two of us", she said.
"I didn't know if it was firecrackers or a auto going off or something", he said.
Macleod said some came to the building in search of other family members.