One protagonist in the shock killing was a Polish immigrant, a father of four who, in his late 40s, decided to pursue his father's calling and become a police officer. His name was Eric Czapnik, 51.
The other, alleged to have ambushed and stabbed Mr. Czapnik in Ottawa early Tuesday morning, is also a policeman - an RCMP officer from Saskatchewan with 11 years' experience and an unstable past, on paid medical leave following brain surgery and a criminal conviction two years ago.
His name is Kevin Reid Gregson, 43.
From what's known, the two men had never met until around 4:30 a.m. Tuesday, on a bitterly cold morning outside the emergency department of Ottawa Hospital's Civic Campus, where Constable Czapnik sat in his cruiser transcribing notes in an unrelated case.
There was no indication of a motive.
Just what Mr. Gregson was doing on that deserted street is one of many questions being asked as Ottawa homicide detectives probe the first killing of an on-duty Ottawa officer in more than a quarter century.
But when arrested and charged with pulling a knife and threatening to kill a bishop of the Mormon Church in Regina three years ago, Mr. Gregson offered a glimpse of his abilities.
"You don't know how many ways I have been taught to kill a man," the prosecutor quoted him as saying at his subsequent trial. "I am better with a knife than a pistol."
Television pictures from the Ottawa crime scene showed both - a handgun that looked like a pellet pistol and two knives, one bloodstained, along with a notebook and a glove. Also behind the yellow police tape was a silver Honda Civic.
Constable Czapnik's killer appears to have been waiting for him, in an attack that culminated in the officer being stabbed in the neck, perhaps through the cruiser's open window.
Mr. Gregson was arrested nearby after being restrained by paramedics and bystanders.
Last night, Ottawa police charged Mr. Gregson with first-degree murder.
The Civic Campus sits a few blocks east of the Royal Ottawa Mental Health Centre, a specialized hospital for the research and treatment of mental illness. Whether Mr. Gregson visited either hospital was not known.
The slaying sent shock waves through the 1,800-plus uniformed and civilian members of the Ottawa Police Service, which had not lost an officer in the line of duty since 1983, when Constable David Utman was gunned down at a shopping centre.
Constable Czapnik had been with the force less than three years.
The son of a career Polish police officer, he immigrated to Canada in 1990 and held other jobs before joining the Ottawa force. Chief Vern White described him as "an engaged and dedicated police officer who truly enjoyed working in the community … well liked by his fellow officers."
The killer, Chief White said, appeared to be "hell bent" in what he was doing.
Mr. Gregson's medical condition may prove central to his defence when a homicide charge is laid.
After pleading guilty in Regina to making a death threat to Robert Howie, a bishop in the city's Church of Jesus Christ of Latter-day Saints, he received a conditional discharge.
The court was told he became incensed and waved a knife after Bishop Howie refused to grant him a special card allowing Mormons to enter the church's temples for certain ceremonies.
Several months after his plea, Mr. Gregson underwent surgery to remove cysts in his brain.
"The medical condition played a large role - the actual medical condition itself, but also the delay in getting it properly diagnosed and the delays in our systems in getting in for MRIs and the follow-ups," his lawyer, David Bishop, told the CBC in April, 2007, following the sentencing.
Cysts and brain tumours can cause changes in behaviour and cognitive ability, said Natalie Lucchesi, a social worker with the American Brain Tumour Association. "There can be anger, aggression. … There could be a paranoia, there could be anxiety and worry. Memory [loss]is a big one," she said.
A report said Mr. Gregson subsequently went through a native addictions-counselling program at Sault Ste. Marie, Ont., and became an orderly at a mental hospital there.
However, he remains a member of the RCMP, a police source said.
He was expected to appear at Ottawa's downtown Elgin Street courthouse later Tuesday or possibly Wednesday.
Constable Czapnik died about an hour after the 4:30 a.m. attack, as paramedics and hospital staff struggled to save his life. Two paramedics also helped apprehend the suspect, Chief White said.
"[It was]not what I'd expect from paramedics, they're not trained for this," he said.
"It is tough," the chief said. "My condolences go out to the family and friends of this officer. And the family of the Ottawa Police Service, because very quickly you become a family member in this organization, as I found out after a couple of years."
The Czapnik family issued a statement saying: "This is the most difficult time for our family and we would like the media and the public to respect our privacy in this time."
Bohne Forsberg, a childhood friend of Mr. Gregson, said the suspect was an average, quiet kid from a hard-working, suburban Ottawa family.
"This really blows me away," said Mr. Forsberg, who attended the same church as Mr. Gregson growing up, the Church of Jesus Christ of Latter-day Saints on Prince of Wales Drive in Ottawa. "I am trying to make sense of this but its seems like events and time have eroded Kevin's sensibilities."
The two men reconnected six months ago after more than 20 years without contact, but they never spoke of any troubles Mr. Gregson was going through, Mr. Forsberg said.
"My heart is just breaking for both families," he said.
Prosecutor Anthony Gerein, who handled Mr. Gregson's 2007 guilty plea, declined comment.
"Because he is presumed innocent until further notice, I won't talk about the past case," he said.
"With criminal history and personal history, there are rules constraining these things … and although I presume he will be prosecuted, all this must be left up to Ottawa police and Ontario courts and we must respect that."
With reports from Beatrice Fantoni and Celia Donnelly