American officials are searching databases and communications for any links to American-based jihadists, law enforcement officials said.
They spoke the same day Canadian lawmakers returned to work after the shootings, giving a standing ovation to the ceremonial Parliament official credited with taking down the gunman who killed a soldier and shook the Parliament area.
The legislators' return came as the country tried to come to grips with the second killing of a soldier on home soil in three days, and questions as to why the attacks came and who exactly was behind them.
"We'll be vigilant, but we will not run scared. We will be prudent, but we will not panic. And as for the business of government, well, here we are -- in our seats, in our chamber, in the very heart of our democracy," Canadian Prime Minister Stephen Harper told the House of Commons in Ottawa on Thursday morning.
Lawmakers stood and cheered Sergeant-at-Arms Kevin Vickers, who officials say took down the suspect in the halls of Parliament minutes after the killing of Canadian army reservist Cpl. Nathan Cirillo at a war memorial nearby.
Vickers, who regularly leads a procession into the House as sessions begin, stood with his ceremonial mace and appeared to be emotional during the ovation.
It was a step toward normalcy for a government district that was widely locked down for hours after the shootings at Canada's National War Memorial and Parliament Hill.
Authorities say a man shot and killed Cirillo, who was standing guard at the war memorial on Wednesday morning. The gunman then entered the nearby main Parliament building in downtown Ottawa, where witnesses say shots were fired -- many by security officers -- before he was shot dead, authorities said.
A plainclothes constable who was working security at Parliament was shot in the leg, according to a House of Commons official briefed on the investigation. The injury is not life-threatening, and the constable was treated at a hospital and released, the official said on condition of anonymity.
The gunman was Quebec native Michael Zehaf-Bibeau, a convert to Islam, U.S. officials told CNN, citing information given to them by Canadian authorities.
Canadian investigators haven't provided any possible motives for the shooting. A U.S. law enforcement official told CNN that a connection to terrorism hasn't been ruled out.
The shootings left government workers and others locked inside offices for a large portion of the day while police searched buildings to ensure that no other culprit was loose.
Wednesday's deadly attack was the second on Canadian soldiers this week. On Monday, a convert to Islam who Canadian authorities said was "radicalized" hit two soldiers with a car in Quebec, killing one of them. Police later killed the man.
"When faced with attacks on the country we all love ... I know we will always stand together," Prime Minister Harper said Thursday. "Canadians will not be intimidated."
Canadian authorities had confiscated Zehaf-Bibeau's passport when they learned he planned to go fight overseas, a U.S. law enforcement official told CNN's Susan Candiotti. The official said it was not clear when that happened.
Zehaf-Bibeau, who was born as Michael Joseph Hall in 1982, had a history of drug use before he converted, two sources said.
"In the days to come, we will learn more about the terrorist and any accomplices he may have had, but this week's events are a grim reminder that Canada is not immune to the types of terrorist attacks we have seen elsewhere in the world," Harper said Wednesday.
Police are satisfied that only one person was responsible for Wednesday's shootings, Ottawa Police Chief Charles Bordeleau told Canadian media outlet CTV News on Thursday morning.
Lawmakers pay respects to soldier
Before Parliament reopened, lawmakers gathered outside the memorial -- some holding flowers -- for a moment of silence for Cirillo.
"This was very off the cuff," lawmaker Charlie Angus told CNN on Thursday morning. "I think parliamentarians really just felt that before we walked into the Parliament buildings, we had to pay respect to a young man who gave his life for his country."
Angus said the soldiers' killings this week left the government with plenty of questions.
"The questions we need to ask ourselves (include), 'How are these people getting this crazy ideology that's inspiring them to do these copycat killings?' " he said.
Another question, he said: What can society do to deal with people who find themselves on the fringes? Issues of mental health, he said, need to be addressed.
"We cannot let people like this fall through the cracks and end up doing deranged killings," Angus said.