10,000 F.B.I. terrorism investigations open

Every day, in F.B.I. offices around the country, agents leaf through classified counterterrorism documents on American citizens one last time. They reread informant reports and review surveillance logs. And then they close the case and walk away. It is a weighty decision, one that supervisors closely review. But with up to 10,000 F.B.I. terrorism investigations open at any given time, there is little time for hand-wringing.

The nightclub shooting in Orlando, Fla., in which a gunman killed 49 people, has brought unusual attention to a seemingly mundane process. For 10 months, F.B.I. agents investigated the gunman, Omar Mateen, but closed the investigation after following a standard checklist. F.B.I. supervisors approved the decision. Managers in Washington, who can reverse any decision to close a case, were notified.

Officials said there was nothing particularly remarkable about the decision — until Sunday’s predawn shooting.

Thousands of investigations are opened and closed. Right now, law enforcement officials say, the F.B.I. is investigating 1,000 potential “homegrown violent extremists,” the majority of whom are most likely tied to or inspired by the Islamic State. Fifty to 100 are considered the highest priority.

The number of agents working on terrorism cases is classified and changes with the threat, but across the country it amounts to several thousand, along with countless analysts. Sorting out angry Americans talking tough from would-be terrorists is among their biggest challenges.

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