The five American Muslim terrorists were Yankee Doodle Dandies. They were born, raised and educated in America. But they were not "of" America because they were separate from the whole. Multiculturalism produces separation and division. These Muslim Americans went to Pakistan to fight their fellow Americans in Afghanistan. That is multiculturalism in action.
The journey to Canada began in Africa and continued without interruption, except by glaciers and volcanos, until their descendants arrived.
Homegrown terrorism, that seems like a decent reason to question multiculturalism. Too many ethnic ghettos.