The meeting set off a chain of actions by the police. Investigators obtained a copy of Mr. Arar’s rental lease. After finding Mr. Almalki listed as an emergency contact, they stepped up their investigation of Mr. Arar. At the end of that month, the police asked customs officials to include Mr. Arar and his wife on a “terrorist lookout” list, which would subject them to more intensive question when re-entering Canada.

However, the commission found that the designation should have only been applied to people who are members or associates of terrorist networks. Neither the police nor customs had any such evidence of that concerning Mr. Arar or his wife, an economist.

From there, the Mounted Police asked that the couple be included in a database that alerts United States border officers to suspect individuals. The police described Mr. Arar and his wife as, the report said, “Islamic extremists suspected of being linked to the al Qaeda movement.”

The commission said that all who testified before it accepted that the description was false.

According to the inquiry’s finding, the Mounted Police gave the F.B.I. and other American authorities material from Project A-O Canada, which included suggestions that Mr. Arar had visited Washington around Sept. 11 and had refused to cooperate with the Canadian police. The handover of the data violated the force’s own guidelines, but was justified on the basis that such rules no longer applied after 2001.

In July 2002, the Mounted Police learned that Mr. Arar and his family were in Tunisia, and incorrectly concluded that they had left Canada permanently.

On Sept. 26, 2002, the F.B.I. called Project A-O and told the Canadian police that Mr. Arar was scheduled to arrive in about one hour from Zurich. The F.B.I. also said it planned to question Mr. Arar and then send him back to Switzerland. Responding to a fax from the F.B.I., the Mounted Police provided the American investigators with a list of questions for Mr. Arar. Like the other information, it included many false claims about Mr. Arar, the commission found.

The Canadian police “had no idea of what would eventually transpire,’’ the commission said. “It did not occur to them that the American authorities were contemplating sending Mr. Arar to Syria.”