“Short of a police state on East German lines the number of such individuals who can be subject to very intensive surveillance sufficient to detect preparations for violent action is but a small proportion of the total — and of course individuals can flip quickly even where they have been checked out previously.” http://www.nytimes.com/aponline/2013/05/31/world/europe/ap-eu-tracking-terrorists.html?ref=world
“This is an area where the power of modern technology and traditional human intelligence and tradecraft need to be melded together, so that incidences of behavior that are not immediately apparent in isolation can be identified as part of a larger pattern,” the U.S. government has emphasized that local communities are most likely to spot unusual or suspicious behavior, and has encouraged more outreach to communities that might be vulnerable to radicalization. The federal government has led a nationwide suspicious activity reporting campaign and trained local police to identify potential terror-related activities.

He simply didn’t “fit the profile of a jihadist,”

After the 2005 suicide bombings in London that killed 52 people during morning rush hour, a parliamentary report found that at least two of the men had been on the periphery of other surveillance and investigative operations.