The 29-year-old Texas man who authorities say opened fire on an Immigration and Customs Enforcement facility, killing at least one detainee, might have been planning the Dallas attack for weeks in an effort to create "real terror," the FBI said Thursday. Investigators have seized devices and analyzed writings of Joshua Jahn , who law enforcement officials say opened fire from a rooftop overlooking an ICE field office on Wednesday, according to a statement by FBI Director Kash Patel .

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