He said they identified Aaron Driver as the individual in the video.
Police raided the home of Aaron Driver in the small town of Strathroy, some 225 kilometres south-west of Toronto, after receiving credible information, including a "martyrdom video", from United States authorities that he planned an attack, the Royal Canadian Mounted Police (RCMP) said. He had also said the Parliament Hill attack by Michael Zehaf-Bibeau was justified.
Lee said police were still outside and had told her they would likely be there all night. The FBI provided a screen shot and later a video of the masked suspect threatening a terror attack.
Driver planned to carry out a suicide bombing in a public area in an urban center during rush hour, Deputy Royal Canadian Mounted Police Commander Mike Cabana said.
Aaron Driver, 24, died shortly after detonating an explosive device in the back of a cab he had entered in Strathroy, Ontario, on Wednesday.
Police said at the news conference that Driver had not been under constant surveillance, but had been monitored.
The FBI tip included a video, which was shown during the news conference.
Police said Driver was identified as the person in the so-called martyrdom video and that he planned an attack within 72 hours.
After his Liberals unseated the Tories in a general election past year, Prime Minister Justin Trudeau scaled back Canada's participation in the coalition, ordering the withdrawal of Canadian fighter jets but increasing the number of military trainers in Iraq.
Last year, federal authorities were so suspicious Driver might have ties to a terrorist group that he bounced in and out of jails and courtrooms for months, all without any actual charges ever being laid - and he had no criminal record at the time. RCMP also say Driver is associated with Elton Simpson, a man involved in an attempted terror attack in Texas in 2015.
Driver was shot and killed after he left his sister's house and got into a taxi, where he set off an explosive device as police moved in.
"If he had gotten out of that [area] before we got there, the scenario would have ended a lot differently". Brenda Carreiro said. "We're baffled".
Canadian transit agencies were notified of the potential threat and were told to be aware of concerning situations after American officials warned of the possibility of an attack. The sources declined to be identified because they did not have permission to speak to the media. He said such notices are commonly issued after security incidents around the world or if the TTC is advised of threats closer to home. "You will pay for everything you ever brought against us".
US authorities sent a screenshot of Driver clad in his balaclava, before sending the footage to their Canadian counterparts on Wednesday morning.
Driver was arrested previous year for openly supporting ISIL on social media.
Islamic State media said Driver was its "soldier", the SITE Intelligence Group monitoring service said on Thursday.
A court order earlier this year placed restrictions on his movements.
Police arrested Driver in June 2015 but a court granted him a peace bond, requiring he maintain good behaviour for a certain period.