Ottawa Police Services handed off the probe to the RCMP Saturday. The Ontario Provincial Police will independently investigate the incident, including the response of police and security forces.
"[The RCMP] asked the OPP to take on an oversight role the equivalent of what the SIU (Special Investigations Unit) would do in Ontario," Ottawa Police Chief Charles Bordeleau said in an interview with CBC News.
Bordeleau said his force is considered too close for such an investigation because his officers were involved in collecting forensic evidence from Parliament and the National War Memorial, where Cirillo was standing on ceremonial guard when he was gunned down by 32-year-old Michael Zehaf-Bibeau.
Zehaf-Bibeau then made his way up Parliament Hill and charged through the doors of Centre Block and engaged in a shoot-out with security inside the building.
The OPP probe will try to answer a wide range of questions about the day's events, including if Zehaf-Bibeau's death was preventable.
Sources said "nearly 20 shots" were fired into Bibeau and about 50 shots were taken by RCMP and House of Commons security over the course of the gun fight.
The RCMP and OPP were not immediately available for comment regarding the investigation.
Gunman wanted to study Qur'an in Saudi Arabia
On Friday, the RCMP said that Zehaf-Bibeau was not among the 93 "high-risk" individuals being monitored as potentially violent radicals, nor was he linked to Martin Rouleau, a man who ran over two Canadian soldiers with his car in Quebec two days before the shooting in Ottawa.
According to his mother, Zehaf-Bibeau was "mad and felt trapped" and had wanted to travel to Saudi Arabia to study Islam and the Qur'an — not Syria, as police have said.
RCMP Commissioner Bob Paulson said during a news conference on Thursday that Zehaf-Bibeau had told his mother he intended to travel to Syria. The information was apparently gleaned from an RCMP interview with Susan Bibeau.
In a written statement published in Saturday's National Post, Bibeau said she had tried to correct the misinformation with RCMP, who said they had no record of the request.
"On the day of the press conference, the investigators were going back and listening and transcribing the audio of the interview with the mother and they realized she was talking about Saudi Arabia," RCMP Deputy Commissioner Mike Cabana confirmed to CBC News.
"They briefed everyone, but it was too late. The Commissioner had already made the statement."
Cabana said realizing the mistake didn't make much of a difference for investigators.
"Individuals who want to travel to Syria often travel to other points like Turkey, or Saudi Arabia, or elsewhere and then transit to Syria, and often don't tell people they plan to go there. So for us, it made no difference," he said.
Canadian officials vowed on Friday to toughen laws against terrorism in the wake of the attack, though critics warned against moves that would curtail civil liberties in a country that prides itself on its openness.