Chief Spy Admits Small, Boston Bombing-Style Plots Beyond the Reach of Surveillance

Minimal communications and civil liberties protections leave little with which to work

With such domestic plots, Clapper said, there are sharp limits on what the 17 intelligence agencies that report to him can do to collect information. The younger brother, Dzhokhar, was a U.S. citizen, and Tamerlan's residential status made him what's known as a "U.S. person," giving both of them protection from surveillance by the FBI and other intelligence agencies.

"There are restrictions on how intrusive we can be in monitoring U.S. citizens," Clapper said. He cautioned that more aggressive efforts to connect the dots could create serious civil liberties issues, and he asked pointedly: "Does the public want us to be more intrusive in monitoring their Internet activity, listening in to their cellphone calls, monitoring their travel overseas? Do you want us to do this to you?"