The Americans were able to return Mr. Arar to Syria after receiving inaccurate information from the RCMP, and on the basis of their own intelligence. This would seem to indicate that Mr. Arar was linked to terrorist organizations or that there was very good reason to believe he was. In any event, these reasons were sufficiently serious to deport him to Syria.
Even though they may have had their reasons, there is no excuse for the Americans not sharing them with Canadian authorities. If two countries engaged in the fight against terrorism exchange information and if one country has information that closely links a citizen from the other country to terrorism -- enough information to deport that citizen to Syria -- then it's natural that it would share that information with the other country. Yet, we are still in the dark as to the reasons involved.