Even by the Mounties’ tight-lipped standards, remarkably few facts were released on Friday about the plot and the people the police said had originated it.

At a news conference, an official of the Mounties, Chief Superintendent Michael LeSage, said that the national police force had received a tip from the F.B.I. in late December that a terror plot was underway in Kingston. About 300 people and a special, low-flying surveillance airplane — which provoked curiosity and irritation among many people in the city — were brought in to aid in the investigation.

Superintendent Peter Lambertucci of the Mounties, who heads one of its counterterrorism units in Ottawa, said that while Mr. Alzahabi and his friend had developed an “attack plan,” they had no target or date. And while the police seized materials that could be components for a bomb, they said the two had not built one. The police would not answer questions about a possible motive or ideology fueling the plot.

Canada’s welcoming attitude toward refugees from Syria’s civil war, which is approaching its eighth year, became a powerful symbol of its openness as a country.

In 2017, Mr. Trudeau famously wrote a tweet saying that Canada was ready to welcome “those fleeing persecution, terror & war.” It came after President Trump moved to ban travelers from several Muslim-majority countries and seemed calculated to present liberal Canada’s embrace of refugees as a counterpoint to the attitude of the Trump administration.

A group of four churches in Kingston sponsored Mr. Alzahabi’s family, said Alex Pierson, the executive director of the Anglican Diocese of Ontario. Citing privacy reasons, he declined to say when the family arrived, other than that it was more than a year ago.

An annual report for one of the churches indicates that Mr. Alzahabi arrived from Kuwait with his father, Amin Alzahabi, his mother and three siblings after their home in Damascus, Syria, was destroyed. Amin Alzahabi, the report said, had been jailed for political reasons and “would be vulnerable to arrest and ‘extreme measures’ should he and the family return home.”