The shooter who opened fire at an Immigration and Customs Enforcement detention facility in Dallas spent a “significant” amount of time planning the attack on ICE personnel and left a note saying he wanted to “maximize lethality,” officials said on Thursday.

A preliminary investigation of the incident that left one ICE detainee dead and two others injured found that the shooter, 29-year-old Joshua Jahn, left a “collection of notes” behind, giving some insight into his thinking.

In the notes, Jahn expressed very negative views of ICE as well as the federal government, Nancy Larson, the acting US Attorney for the Northern District of Texas, said in a press conference.

He also said he acted alone in the attack, wanted to “maximize lethality” against ICE agents, and hated the federal government.

“He hoped his actions would give ICE agents real terror of being gunned down,” Larson said. “He hoped his actions would terrorize ICE employees and interfere with their work which he called ‘human trafficking.’”

Officials said Jahn spent time pre-planning the attack by researching the ICE facility and tracking agents using an app.

As part of the planning, the shooter drove to the Dallas ICE facility with a ladder, which allowed him to reach the roof of a nearby building. He used a legally obtained 8-mm bolt-action rifle to open fire, spraying bullets across the ICE facility.

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