Investigators believe one of the two suspects charged in Canada with plotting to blow up a passenger train travelled to Iran within the past two years.
US law enforcement and national security officials said Chiheb Esseghaier's trip is relevant to their investigation but are not disclosing details, Reuters reports.
The Tunisian is jointly charged with Raed Jaser, who has been identified as a Palestinian with United Arab Emirates citizenship.
The pair were arrested on Monday after a joint Canada-US investigation that started last year, based on a tip from a member of the Muslim community.
Canadian authorities said the men were supported by al Qaeda elements Iran, a claim rejected by Iran's Foreign Minister.
Canadian police say there is no sign of Iranian government involvement with the suspects.