The veteran FBI agent and a local sheriff’s deputy took no chances when they got a credible tip about a potential terrorist. In a joint operation, they ran his name through a maze of federal criminal and terrorism databases and scrutinized his telephone records for suspicious contacts. Without a warrant, they couldn’t read his emails or listen to his calls. But they watched him from unmarked vehicles to track his daily routine and to see whom he met. They deployed two confidential informants more than a dozen times to secretly record his conversations. They interviewed him twice and convinced him...