Napolitano responded that Tsarnaev's departure did "ping" in the homeland security screening system, but she noted that because the FBI's investigation in 2011 found no suspicious activity, there was no reason to follow up.

CNN: Bombing suspect not on terror watch list

Security screening has tightened, as Congress debates how to revamp the system while attracting a higher-quality, better-trained work force.

CNN: The question remains: Can we ever be safe?

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.

ENGADGET: Minority Report at 10: a look at technology from today to 2054