In late September 2001, CIA director George Tenet phoned Michael Hayden, then director of the National Security Agency, to ask a fateful question. The NSA had the intelligence community’s fattest budget, an array of the world’s most powerful computers, and the benefit of operating in near-complete secrecy. The president and vice president, Tenet told Hayden, wanted to know if there was more his agency could be doing to prevent the next attack.

“No,” Hayden said, “not within my current authorities.”

Tenet paused. “That’s not actually the question I asked you. Is there anything more you could do?”