London attacks raise concerns over the potential for sleeper cells of Americans. When security cameras captured four young Britons sauntering into the London Underground before detonating their deadly backpacks last month, the chilling images raised questions about whether such homegrown sleeper terrorists could be plotting attacks in the United States. U.S. counter-terrorism officials say there is no evidence that such would-be terrorists exist in large numbers in the United States, or that any of them are in the operational stages of a plot. And some U.S. officials and experts downplay the threat such domestic militants might pose to Americans. Full Story