While I would not dream of using arguments to diminish the horror of the September 11 attack for thousands of people, I would also suggest that the people who died in the attack did not suffer more terrible deaths than animals in slaughterhouses suffer every day. Moreover, the survivors of the September 11 attack and their loved ones have an array of consolations-patriotism, the satisfaction of U.S. retaliation, religious faith, TV ads calling them heroes, etc--that the chickens, whose lives are continuously painful and miserable, including being condemned to live in human-imposed circumstances that are inimical and alien to them as chickens, do not have available. They suffer raw, without the palliatives. Doubtless the majority, if not every single one, of the people who suffered and/or died as a result of the September 11 attack ate, and if they are now alive continue to eat, chickens. It is possible to argue, using (Peter Singer's) utilitarian calculations, that the deaths of thousands of people whose trivial consumer satisfactions included the imposition of fundamental misery and death on hundreds of thousands of chickens reduced the amount of pain and suffering in the world.
In conclusion, I think it is speciesist to think that the September 11 attack on the World Trade Center was a greater tragedy than what millions of chickens endured that day and what they endure every day because they cannot defend themselves against the concerted human appetites arrayed against them. Perhaps the word "tragedy" should not be used anyway in this context unless in the more precise sense of a fundamentally terrible thing happening to a human being who consciously or subconsciously brought the terrible thing upon him or herself, lived through it, and gained insight and wisdom as a result. In this classical sense of tragic drama, it remains to be seen whether America is a "tragic hero" or even a "tragic" victim. If, though, the question is whether the World Trade Center attack was worse for its thousands of human victims than the sum total of misery and terror was for millions of chicken victims that day, I see only one nonspeciesist answer to the question.