Many questions remain unanswered about how the FBI and CIA handled the information given to them about Boston bombing suspect Tamerlan Tsarnaev.
Russia first identified Tsarnaev as a possible extremist in 2011. Russian officials requested information on him, first from the FBI and then the CIA.
But the FBI ultimately concluded he posed no threat, after an investigation that included interviewing Tsarnaev and his family. They placed Tsarnaev's name on two terrorist watch lists,that would alert authorities if Tsarnaev traveled overseas.
But Tsarnaev was still able to leave the country and fly to Dagestan and Chechnya, despite all of these precautions. And authorities were unable to use the information they had to prevent the bombings in Boston.
Tim Weiner, author of "Enemies: A History of the FBI" and "Legacy of Ashes: The History of the CIA" explains how terrorist threats are identified, and how information is, or is not shared across federal agencies.