MacCoubrey said as many as 17 more shots rang out between that time and around 8:30 a.m.
On Thursday, Ontario pledged more money for police and to keep suspects behind bars while they await trial on gun crimes charges, as the Canadian province grapples with rising shootings involving domestically obtained weapons.
Costello, a detective known as Robb, worked on all manner of cases - from missing persons, robberies, street crimes, extortion, sex crimes - in Fredericton police's major crimes unit and while seconded to the RCMP's Internet child exploitation unit. She is survived by her husband and three children.
A police tweet said: 'Fredericton Police are responding Brookside Drive area.
Witnesses described to the Canadian Broadcasting Corporation (CBC) seeing the barrel of a rifle or shotgun poking out from the window of a low-rise apartment building and firing into a courtyard, while the bodies of two officers lay on the ground nearby. "Being on lockdown was very freaky because I leave my door unlocked most of the time".
Police in Fredericton are asking anyone with photos or video footage of the fatal shooting in Fredericton to come forward.
The area he's called home for a year, which features houses, a school and other hallmarks of a peaceful residential community, is the last place Hrubeniuk said he expected to encounter a unsafe situation.
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Fredericton, the capital of in New Brunswick province, has a population of about 60,000.
Police have stated that they will release confirmed information as soon as they can.
Two civilians, a man and a woman, were also killed in the shooting.
He called 911, heard more shots and took another glance out the window. "They aren't letting customers in at the moment", he said.
New Brunswick Premier Brian Gallant tweeted his condolences to the victims and their families. "In this time of shock and grief, let us all protect their families", Fredericton's mayor, Mike O'Brien, said on Twitter.
Fredericton police were calling on residents in the area of the shooting to stay in their homes with the doors locked.
A monument set up by residents of Fredericton at the headquarters of the Fredericton Police Force on Friday afternoon.
About 50 people from the apartment complex were displaced after the shooting, and police said the city and Red Cross were arranging alternate lodging for the residents.
- With Michelle McQuigge and Gabriele Roy in Toronto, Aly Thomson and Keith Doucette in Halifax, and Holly McKenzie-Sutter in St. John's.