President Barack Obama said he was rattled by it.
There are images of him on his first foreign trip as president, laughing with Prime Minister Stephen Harper as they walked down that Gothic-style ceremonial hallway in February 2009.
"I had a chance to travel to the Parliament in Ottawa. I'm very familiar with that area," Obama recalled Wednesday.
"I am reminded of how warmly I was received and how wonderful the people there were. And so, obviously, we're all shaken by it. But we're going to do everything we can to make sure that we're standing side-by-side with Canada during this difficult time."
On Wednesday, he called Harper to offer his condolences over what the president described as a tragedy and an outrage. While he said the details remained unclear, Obama said it's very important for Canada and the U.S. to remain "entirely in-sync" when dealing with terrorist activity.
The shooting led to increased security at the U.S.'s Ottawa embassy. Given that the attack began at Canada's tomb of the unknown soldier, there was also stepped-up security at the American equivalent of that monument, in Arlington, Va.
The White House daily briefing began with a word of condolence.
Britain's Prime Minister David Cameron also walked down that hallway that separates the two chambers of Canada's Parliament on Sept. 22, 2011 — also smiling, with Harper beside him.
He took to Twitter to express his indignation: "I'm appalled by today's attack in Ottawa. I offer my full support to @pmharper and the Canadian people as they deal with this incident."
Benjamin Netanyahu made that walk with Harper seven months ago. The Israeli prime minister's office tweeted its own statement of condolence, and expression of support.
Australia's Tony Abbott and New Zealand's John Key, who have both visited that spot, also offered their condolences and pledged solidarity with Canada.
The shooting was a top news story in numerous countries.
All the main American news networks offered live coverage throughout the day. It was also a lead item on the best-known newspaper sites in a number of countries including the U.S., the U.K., Mexico, France and Italy.
The top headline on France's Le Monde newspaper website was, "Ottawa Boucle" (Ottawa On Lockdown). Italy's La Repubblica website ran several headlines underneath one that declared, "Canada Sotto Attacco" (Canada Under Attack).
Even in Turkey — which borders an active war zone where an international coalition is fighting Islamist rebels — the Canadian events became major news.
Some foreign media instantly drew conclusions about a terrorist link to the attack — this in the absence of any official word yet from authorities about a motive.
The U.K.'s Daily Telegraph asked, "How An Oasis Of Tranquility Became a Breeding Ground For Terrorists." America's Daily Beast declared in a headline, "Terrorist Ends Canada's Innocence."
One Canadian MP told an international audience that, on the contrary, the country must resist being bullied into changing.
NDP MP Charlie Angus was one of numerous Canadian politicians who granted interviews to all-news U.S. cable outlets, as they offered blanket coverage throughout the day.
He described hearing the gunshots from inside the Opposition members' chamber during the weekly caucus meeting, right across that hall of honour from the government caucus room.
And he told his American interviewer that Canadians weren't going to be intimidated by this.
"We're not gonna let punks, or crazies, or terrorists, take us down. Not gonna happen," the NDP MP told the Fox News Network.
"We don't give in to it."
One U.S. law-enforcement analyst told CNN that this incident would produce the same kind of scrutiny in Canada that occurred when a knife-wielding man recently jumped the fence to enter the White House.
That White House invasion prompted a series of follow-up stories about other security lapses, which led to the resignation of the head of the Secret Service.
A recurring theme of some of the American news coverage was that question of security: How could a gunman manage to walk right into Canada's Parliament?
In Washington, after the 9-11 attacks, security tightened considerably. The effects remain evident to this day. Airport-style metal detectors were installed in numerous buildings and they remain there, including at Canada's embassy.
But not even those safeguards, nor the fact that the main stretch of Pennsylvania Avenue has been shut down to vehicle traffic since the 1990s, prevented an intruder from recently entering the White House.
In response to that incident, the Secret Service recently set up a new, movable, fence outside the White House, just a little farther out from the older permanent one.
Someone jumped that fence late Wednesday, in a strange coincidence of timing. He was seen by witnesses kicking a police dog on the White House lawn, then arrested.
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