The Ottawa police response to the shooting involved 310 officers at its height, dozens of whom deployed themselves when they heard about the fatal shooting of ceremonial guard Cpl. Nathan Cirillo at the National War Memorial and the storming of Parliament Hill just moments later.
It was the largest reactive deployment of officers to an unplanned emergency in the force's history.
But the sheer scale of the police response contributed to the confusion of the investigation, according to Insp. Mark Ford of the Ottawa police emergency operations directorate.
On Oct. 22, Ford was one of two Ottawa police incident commanders managing the force's response to the shooting.
"First responders want to do the right thing," Ford told the CBC's Judy Trinh on Thursday. "They self-engage, they go to the scene, they want to help … and the challenge there is how do you manage them effectively and, also equally important, how do you account for all of your officers so you know where each and every officer is?
"Given the scale and the number of officers involved it was something that was a challenge for us, and what we have to look at in the future is, can we do it better?"
Officers in balaclavas caused concern
Some of the police who responded to the shooting wore balaclavas to conceal their identities because they normally work in undercover units.
Others were in various states of uniform that confused members of the public who, for most of the day, didn't know how many active shooters or other suspects might be involved.
In the future, the force wants officers who self-deploy to make sure they can be clearly identified as police.
"If you see a person walking around with a balaclava on, with no markings, carrying a firearm, it's quite easy — especially in that state of crisis — that people might perceive ... maybe this person isn't a police officer and it's someone that's going to inflict harm on me," Ford said.
8 hours to confirm 1 shooter involved
It took only an hour for police to determine that gunman Michael Zehaf-Bibeau was the man responsible for shooting and killing Cirillo and, moments later, opening fire inside Centre Block on Parliament Hill.
But it took RCMP and Ottawa police about eight hours to confirm that Zehaf-Bibeau was acting alone, Ford said.
Part of the reason was the conflicting eyewitness reports of multiple shooters and suspects, all of which had to be checked out by investigators.
Police sources told CBC News that some witnesses reported seeing a second gunman run into bushes at the National War Memorial, but that was determined to be the driver of a cabinet minister's vehicle, who ran to hide after the armed Zehaf-Bibeau commandeered the vehicle.
Some witnesses also reported seeing multiple people in Zehaf-Bibeau's vehicle, and others reported seeing what they thought were two suspects with long guns running near the National War Memorial, sources said. The second person turned out to be the other sentry who was standing guard with Cirillo.
There was also a 911 call reporting a third shooting scene at a downtown mall, which police reported to the public and later retracted.
"As a police agency we had to ensure that the community wasn't at risk and we had to investigate those other leads ... to ensure that there wasn't a second, third shooter involved in this incident," Ford said.
"All of this contributed to confusion, and it takes a significant amount of time to eliminate that there are no more threats and to announce to the community that things have returned to a state of normalcy."
Communication problem between forces
There were communication problems between Ottawa police, RCMP and Parliament Hill security personnel at the scene who were using different radio frequencies.
Concerns have been raised repeatedly over the years about the different silos in which law enforcement and security officials operate on and around Parliament Hill, including by Auditor General Michael Ferguson, who reported on them in 2012.
On Oct. 22, RCMP and Ottawa police tactical officers used hand signals to communicate while they were sweeping and clearing buildings on Parliament Hill.
To remedy the problem, Ottawa police say they've ordered new radios that will allow communication with other first responder radios during emergency operations.
Ottawa police have also ordered new night-vision goggles, after the force's tactical officers found that the current goggles didn't work well as they were clearing buildings on the Hill, and that there weren't enough of them.