Although one person held in ICE custody was killed and two other detainees were critically wounded in Wednesday's bloodshed, it seemed clear from the gunman's writings "he did not intend to kill detainees or harm them," Nancy Larson, acting US attorney for the Northern District of Texas, said at a news conference.
No government personnel were injured in the incident, though officials said ICE agents and other federal officers rushed into harm's way to save some detainees sitting helpless in transport vans while shots were being fired.
The perpetrator was found dead from a self-inflicted gunshot on the rooftop of a nearby building from which he fired on the ICE building and transport vans parked in its entry with a bolt-action rifle, officials said.
The weapon was legally purchased by the gunman in August, according to authorities.
The suspect was identified as Joshua Jahn, 29, a Dallas-area resident who previously attended a community college and had worked as a solar panel installer.
He had climbed to his rooftop sniper's perch using a ladder carried to the scene atop his car, Larson said.
His writings were discovered during a search of his home in Fairview, Texas, Larson told reporters.
"Yes, it was just me and my brain," she quoted one of his notes as saying, adding the messages showed a "game plan" for the attack.
"He hoped his actions would terrorise ICE employees and interfere with their work, which he called human trafficking," the prosecutor said. "What he did is the very definition of terrorism."
Joseph Rothrock, special agent in charge of the FBI field office in Dallas, said all indications were that the shooter "committed this act alone," as stated in his own words.
"His handwritten notes show he did not expect to survive this event," Rothrock said.
President Donald Trump and others in his administration say the incident proves an increase in vitriolic rhetoric directed against ICE, the primary enforcement agency of Trump's aggressive immigration crackdown, is putting law enforcement at greater risk.
They also cast blame on the availability of apps capable of tracking the location of ICE agents.
According to FBI officials, Jahn used ICE-tracking apps and downloaded a list of local US Department of Homeland Security facilities in preparation for Wednesday's pre-dawn attack.
The suspect also had researched video of conservative activist Charlie Kirk's highly publicised assassination, FBI Director Kash Patel said on social media on Thursday.
The investigation thus far, Patel said, "indicates a high degree of pre-attack planning."
One of the first glimmers of the gunman's motives to be made public was a photo the FBI released within hours of the shooting of an unused bullet found inscribed with the phrase "ANTI-ICE."
Wednesday's attack was the third shooting this year in Texas at a DHS facility. A police officer was shot in July at an ICE detention center in Prairieland, and a Michigan man was shot dead by agents after opening fire on a US Border Patrol station in McAllen in July.
On his Truth Social platform, Trump accused "Radical Left Democrats" of stoking anti-ICE violence by "constantly demonising Law Enforcement, calling for ICE to be demolished, and comparing ICE Officers to Nazis."
On Thursday, Trump signed a presidential memo seeking to crack down on what he has characterised as organised efforts by left-wing groups to commit or incite political violence.
