Ramsay Talks, the organizer of the event Khan was to speak at, seemed to take Khan at his word on Monday and included a statement from him in a cancellation post on Facebook. "This turn of events is not just of deep concern to me but to all my fellow Americans who cherish our freedom to travel abroad," said Khan, according to the post. "I have not been given any reason as to why. I am grateful for your support and look forward to visiting Toronto in the near future."

The claim, which does not state which U.S. agency contacted him, immediately raised doubts about how it was possible that a U.S. citizen was being prevented from traveling abroad.

On Tuesday, Bob Ramsay, who runs Ramsay Talks, said he didn't know the specifics of Khan's predicament. "I don't know exactly who conducted the review, but in speaking with Mr. Khan, it was certainly U.S. authorities," Ramsay said. "That's all I know."

As questions about his motivations for making the claim swirl, Khan has refused to elaborate on his initial statement to The Washington Post and other publications. A more detailed request for clarification did not receive an immediate response Tuesday afternoon.

It is unclear whether Khan has previously traveled outside the United States since he was naturalized.

U.S. citizens don't need visas to enter Canada, or even the electronic travel authorizations required of all other foreign visitors there. As a general rule, the United States cannot prevent passport-holding citizens from traveling if they have not been charged with a crime. Public records indicate that Khan has no criminal history, either at the federal level, in Charlottesville, where he lives, or in Silver Spring, his previous place of residence. Furthermore, U.S. Customs and Border Protection told Politico that, as a rule, it does not contact travelers before their trips.

The Canadian foreign ministry also denied issuing any review of Khan's ability to travel there.

"We are unaware of any restrictions regarding this traveler," said Camielle Edwards, spokeswoman for Canada's Immigration Minister Ahmed Hussen.