U.S., Canadian and U.K. officials believe that a Ukrainian commercial aircraft that crashed shortly after takeoff from Tehran on Wednesday, killing all 176 people on board, was downed by a missile system fired by Iran, possibly by mistake.

“We have a high level of confidence that this was shot down by Iran,” one U.S. official said Thursday. The official said the plane was being tracked moments before it went down by Iranian radar used to aim missiles. A second official said the U.S. believes Iran may have downed the aircraft...