Napolitano responded that Tsarnaev's departure did "ping" in the homeland securityscreeningsystem, but she noted that because the FBI's investigation in 2011 found no suspicious activity, there was no reason to follow up.
As revealed last year, the US Department of Homeland Security is working on a system called FAST (Future Attribute Screening Technology) that relies on an array of sensors and advanced algorithms to detect physiological and behavioral cues that are said to indicate malintent.