Almost every major terrorist attack on Western soil in the past fifteen years has been committed by people who were already known to law enforcement. One of the gunmen in the attack on Charlie Hebdo, in Paris, had been sent to prison for recruiting jihadist fighters. The other had reportedly studied in Yemen with Umar Farouk Abdulmutallab, the underwear bomber, who was arrested and interrogated by the F.B.I. in 2009. The leader of the 7/7 London suicide bombings, in 2005, had been observed by British intelligence meeting with a suspected terrorist, though MI5 later said that the bombers were “not on our radar.” The men who planned the Mumbai attacks, in 2008, were under electronic surveillance by the United States, the United Kingdom, and India, and one had been an informant for the Drug Enforcement Administration. One of the brothers accused of bombing the Boston Marathon was the subject of an F.B.I. threat assessment and a warning from Russian intelligence.

—Mattathias Schwartz, writing in The New Yorker

That’s just the first paragraph.

The NSA wants to collect everything, but they already are collecting more than they need.