Const. Robb Costello would often tell his spouse that no matter what happened to him on the job, he would always come home.

On Friday, the Fredericton officer didn't return.

Instead, police showed up at Jackie McLean's home to tell her the 45-year-old was among four people killed in a shooting that took place in a quiet residential neighbourhood.

"This is the first day he did not come back home," McLean, Costello's common-law partner, told The Canadian Press in an interview. "I am having a really hard time envisioning my life without him."

Costello and Const. Sara Burns were the first officers to arrive at the scene of the early morning shooting. They were shot as they rushed over to two people lying on the ground.

McLean said Costello, a 20-year veteran of the force, had left early for his Friday morning shift, which began at 7 a.m.

"He is always really prompt and on time, and he always says that if you are not early you are late," said McLean, adding that her partner would always leave for work with a smile.

"He loved being a police officer and he lived for being a police officer," she said. "He is the only police officer who I have ever known who could write someone a ticket and have the person thank them after."

Costello had two daughters from a previous relationship, but McLean said he was also a role model for her own two children.

"He is phenomenal as a father, he is very generous with his time and loved my children just as though they were his own," she said. "We were very close to one another; we spent all of our free time together."

Police Chief Leanne Fitch said the force is struggling to cope with the loss of Costello and Burns, a 43-year-old mother of three.

Burns told a newspaper in 2015 that she saw policing as a demanding career.

"(Policing) is something that I think challenges you physically, mentally and socially; you need to be aware of those components," she told the Journal Pioneer newspaper. "It's a really challenging field and to survive you need to keep all of those in check."

Prime Minister Justin Trudeau praised Costello and Burns for their bravery.

"They did not think twice about what they had to do to keep their fellow Canadians safe," Trudeau said in a statement. "We will not forget the two fallen police officers whose sacrifice no doubt saved lives and prevented even greater tragedy."

McLean said Costello had planned to retire in the next six to seven years.

"We were looking forward to building years with just the two of us," she said. "I still am in complete shock, and I don't really quite know how to process it."

Family members were flying in to Fredericton from across Canada on Friday, she said. Wives of other police officers had reached out to offer their support, while others had stopped by with food and coffee, she said.

McLean said she didn't know yet when a funeral might be held but she knew what Costello would want.

"Robb was very adamant ... that if something was ever to happen to him he did not have a sad funeral," she said. "He wanted to have a true Irish Wake and wanted to have a party with lots of laughs and happy memories."