Homegrown terrorism is another serious danger. Previously unknown individuals, often with no prior ties to terrorism, “self-radicalize” by viewing violent propaganda online. Like the Tsarnaev brothers, believed responsible for the Boston Marathon bombings, such “lone wolf” terrorists are difficult to detect.

Our recommendations centered on how to protect the country. In recent conversations, however, national security leaders have expressed concerns about relentless cyber attacks against our nation’s most sensitive and economically important networks. Former National Security Agency director Keith Alexander has described the ongoing cyber theft of U.S. intellectual property as “the greatest transfer of wealth in history.” One lesson of 9/11 is that Americans did not awaken to the gravity of the terrorist threat until it was too late. That may be happening again in cyberspace.

The booming scale of government data collection has also changed. Data collection and analysis are vital tools for preventing terrorist attacks. In counterterrorism, no single intelligence report is definitive, and each clue may appear anodyne in isolation. The accumulation, filtering and synthesis of vast amounts of information lead to counterterrorism breakthroughs.