U.S. Security Leaders Detail Terrorist Threats for House Panel --- Terrorists Trained in Iraq, Syria Who Return Home and Americans Radicalized Online Are Top Concerns -- Federal Bureau of Investigation Director James Comey said Wednesday he is worried about "a terrorist diaspora" out of Iraq and Syria as fighters leave extremist groups there and return to their homes -- "I am very concerned about the going. I'm even more concerned about the coming," Mr. Comey said. -- Mr. Comey, Homeland Security Secretary Jeh Johnson and National Counterterrorism Center Director Matthew Olsen detailed a series of threats before a House of Representatives panel on Wednesday. They said they are concerned about the threat from Islamic State, but warned that the country must grapple with a range of vulnerabilities, including Americans radicalized online who commit terrorist attacks on their own. -- "It's no longer necessary to actually meet somebody in al Qaeda to get training and inspiration to conduct a terrorist attack here in the U.S.," Mr. Comey said. "Somebody can do it in their pajamas in their basement." -- Homegrown terrorists, the officials said, are perhaps the hardest to detect and stop since they don't reach out to others before acting. -- "In a country this big and this free, with the material that's available, it's a big challenge for us," Mr. Comey said. -- Still, Mr. Comey and Mr. Johnson said they are making progress on two fronts: combing online postings to identify potential threats from within the U.S. and working with communities that might breed radicalization. -- Throughout the two-hour hearing, the officials found themselves repeatedly responding to Republican lawmakers' contentions that terrorists could be infiltrating a porous southern border. -- Messrs. Johnson and Olsen said repeatedly that they have no intelligence or evidence to suggest such a plot. -- Mr. Johnson did confirm the thrust of a Wall Street Journal report that the Department of Homeland Security plans to install a Pentagon-like regional command structure over its agencies operating along the southern border. He said he plans to announce in October details of "an effort to more strategically bring to bear all the resources of my department on border security." - More, Andrew Grossman, WSJ, http://online.wsj.com/articles/u-s-security-leaders-detail-terrorist-threats-for-house-panel-1410984249