WASHINGTON — The U.S. was slow to take seriously the threat posed by homegrown radicals and the government has failed to put systems in place to deal with the growing phenomenon, according to a new report compiled by the former heads of the Sept. 11 Commission. The report says U.S. authorities failed to realize that Somali-American youths traveling from Minnesota to Mogadishu in 2008 to join extremists was not an isolated issue.