CBS News reported that police in the city of Fredericton, the capital of New Brunswick, asked residents to shelter in place on Friday morning.
David MacCoubrey, who lives in Fredericton, said he heard about 20 shots in total.
The police urged people to stay inside and lock doors.
Fredericton police say they will not be releasing the names of the shooting victims at this time.
He said police have been searching the complex, and he's been sitting away from windows.
Genine Patricia Paul-Dimitracopoulos met Constable Costello almost 15 years ago in her role as a social worker with St. Mary's Child and Family Services.
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The crime scene is contained and officers expect to be working on the investigating for some time.
"I thought, 'Maybe it is something a little bit more than just noise, '" he said. They have searched all the apartments in the building.
"The next few days will be a challenging time for our community as we deal with a loss that will no doubt be felt by many".
Two police officers, Const. Lawrence Robert Costello and Const. Sara Mae Helen Burns, were among those gunned down in the early morning shootout. "It sounded like it started in the courtyard area".
Between 1961 and 2009, a total of 133 police were killed, and just five in the province of New Brunswick.
Police officers were initially called to the Brookside Drive shortly after 7 a.m. after receiving a report of shots fired in the area. There was basically three dead people laying there.
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For some, the terror began from the moment they opened their eyes.
"I hear more shots and looked out and there's two police officers on the ground".
"We are all shocked and saddened to learn this morning of the ongoing tragic incident in Fredericton", Kenney said, calling it a "reminder of the risks our fearless men and women in uniform are willing to take every day to keep us safe".
Canadian public broadcaster CBC said that the shooting happened in a residential area of the city north of downtown, and that heavily armed police, paramedics and firefighters were on the scene.
In June 2014, RCMP Constables Doug Larche, Fabrice Gevaudan and Dave Ross were killed in a shooting rampage by Justin Bourque.
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