The gunman who killed one detainee and wounded two others at an Immigration and Customs Enforcement field office in Dallas was trying to target ICE personnel and appeared to be planning his attack for months, authorities said Thursday.

The shooter, identified as 29-year-old Joshua Jahn of Texas, "specifically intended to kill ICE agents" in Wednesday's ambush, said Joe Rothrock, the special agent in charge of the FBI's Dallas field office, at a news conference.

Rothrock said Jahn's plans began "months in advance." The special agent noted that Jahn obtained a bolt-action rifle in August and pointed to his research on Homeland Security facilities.

The strongest evidence of motive came from Jahn's own words in a collection of notes found when FBI agents searched the shooter's residence a

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