Dallas anti-ICE gunman Joshua Jahn harbored deep resentment for both immigration authorities and the federal government, and set out to inflict maximum casualties in an attack officials called “the very definition of terrorism.”

Speaking at a press conference Thursday morning, Acting US Attorney Nancy Larson said hand-written notes found at Jahn’s residence by the FBI referred to the work done by ICE agents as “human trafficking,” and said the Dallas facility he riddled with bullets was nothing but “people showing up to collect a dirty paycheck.”

The gun used by Jahn, who authorities believe acted alone, was an “8MM bolt-action rifle” he legally acquired in August, FBI special agent in charge of the Dallas Field Office Joe Rothrock told reporters.

Larson said authorities believe Jahn drove to the building where he took up his sniper perch before the shooting around 3 a.m. Wednesday, toting a “large ladder” on the roof of his car.

Investigators said they believe he used this ladder to access the rooftop of the immigration attorney building from which he would later rain bullets upon the ICE facility, fatally shooting one detainee and critically wounding two others.

Here's what we know about the Dallas ICE facility shooting

• A gunman identified as 29-year-old Joshua Jahn opened fire on an ICE van at a detention center in Dallas, Texas, in the early hours of Sept. 24.
• Jahn began shooting while detainees were being transferred and the portcullis to the facility was open, reports claim.
• One person died and two were wounded, according to reports. Jahn was found dead on a nearby rooftop from a self-inflicted gunshot wound.
• There was writing on the bullets, with one reading “ANTI-ICE,” according to the FBI.
• The shooting is being investigated “as an act of targeted violence,” the special agent in charge of the FBI’s Dallas Field Office, Joe Rothrock, said in a press conference.

Larson said another note said he expressed “hatred” for the federal government, using “crude” language which she did not elaborate on.

Another note written by Jahn implied he acted alone, and referenced the investigation he knew his actions would prompt.

“Yes, it was just me and my brain,” he wrote of the attack. “Good luck with the digital footprint,” he taunted, implying he had deleted data from his devices, Larson said.

Larson called the attack “the very definition of terrorism,” while Rothrock said Jahn’s notes said he “acknowledged the potential for other casualties, and knew with a high degree of likelihood that detainees would be transported that morning at that location.”

Rothrock said when the bullets started flying from an adjacent rooftop, ATF and ICE agents “put themselves in the line of fire” to move individuals out of harm’s way.