TORONTO - Officials on both sides of the border erected a wall of silence Saturday over word that an Afghan immigrant arrested on terrorism-related charges in the United States made at least two trips to Canada.

The reference to Najibullah Zazi's trips to Canada was almost made in passing Friday by U.S. prosecutor Tim Neff during a Denver court hearing.

Zazi is accused of plotting to use weapons of mass destruction to attack commuter trains in New York City to coincide with the anniversary of the Sept. 11 attacks.

They say Zazi was stopped by police Sept. 10 as he entered New York, and he dropped his plans for an attack once he realized that law enforcement was on to him.

Neff did not give any other details on the Canadian trips and a spokesman said in an email later Friday that he had no comment on them.

An FBI spokeswoman said Saturday the agency could not confirm "anything regarding this case at this time."

An aunt who lives in the Toronto-area told the Globe and Mail that Zazi has visited relatives in the area twice "a year or two ago."

The paper also reported Saturday that agents with the Canadian Security Intelligence Service were knocking on the doors of relatives of Zazi in Mississauga, Ont, west of Toronto, Friday night.

But Canadian officials refused to acknowledge that the spy agency was investigating. CSIS spokeswoman Manon Berube said the service does not discuss its operations as a matter of course.

"No comment on specific operations, but we would certainly be co-operating with our American counterparts," she said.

She referred inquiries to Public Safety Minister Peter Van Loan's office, which in turn referred calls back to CSIS for operational security matters.

Van Loan's spokesman Christopher McCluskey refused to speak about the case.

"Canada monitors national security concerns and is vigilant in protecting against any terrorist threats," he responded through email, after numerous attempts to contact him.

"Beyond that, I cannot comment on any individual case before the courts, or any individual file that engages in national security."

But a private investigator in the U.S., who touts himself as a specialist in tracking down suspected terrorists, as well as suspected philanderers, said the potential Mississauga connection is a "red-flag."

"That's terrorist central in Canada," said Bill Warner, a private investigator who boasts he has worked with the FBI on cases involving terrorism.

"If he was in that city, that's one of the key spots in Canada for these terror connections and they link back into Atlanta, Chicago, Toledo," Warner said.

The FBI wouldn't confirm Warner's credentials, saying it could not reveal who the agency works with as a matter of policy.

Zazi is due to appear in court in Brooklyn on Tuesday to face the charges.

Prosecutors say Zazi received explosives training from al-Qaida in Pakistan and returned to the United States bent on building a bomb.

Over the summer, he and three associates went from one beauty-supply store to another in a Denver suburb buying chemicals to make explosives like those that killed dozens of people in transit bombings in London and Madrid, investigators said.

At least three and possibly more of his accomplices remain at large, and investigators have fanned out across New York in pursuit of suspects. Authorities also issued a flurry of terrorism warnings for sports complexes, hotels and transit systems.

Zazi ran a coffee cart in Manhattan before moving to Denver this year and getting a job as an airport shuttle driver.