As nation marks 10th anniversary of 9/11, New York City and Washington, D.C. are on high alert over a 'credible' terror threat that Al Qaeda plans to launch an attack.Al Qaeda 2.0 Behind New Threat? | VIDEO: Officials on High Alert | Full Coverage: 9/11 AnniversaryPALKOT: View of 9/11 From Abroad | Are America's Cyberdefenses Weaker?