Washington DC - The gunman who opened fire on an immigration facility in Dallas apparently acted alone and was seeking to "terrorize" federal agents, US officials said Thursday.

Joshua Jahn (29) died from a self-inflicted gunshot wound after spraying the US Immigration and Customs Enforcement center with gunfire on Wednesday.

One ICE detainee was killed and two others were wounded – but officials say that Jahn's intended target was ICE, the agency chiefly responsible for carrying out President Donald Trump's pledge to expel millions of undocumented migrants.

Nancy Larson, acting US attorney for the North District of Texas, told a press conference that a collection of notes found at Jahn's residence laid out his motivations.

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"It's clear from these notes that he was targeting ICE agents and ICE personnel," Larson said. "He hoped his actions would terrorize ICE employees and interfere with their work, which he called human trafficking... The tragic irony for his evil plot here is that it was a detainee who was killed and two other detainees that were injured."

FBI special agent Joe Rothrock said Jahn apparently put months of planning into the attack and legally purchased the rifle that he used in August.

Jahn opened fire on the ICE facility from the roof of a nearby building, and FBI director Kash Patel published a photo on X of five of his unspent bullets – one of which was marked with the words "ANTI-ICE."

"His words were definitively anti-ICE," Larson said. "That said, we did not find evidence of membership in any specific group or entity."

ICE's prominent role in the Trump immigration crackdown has sparked widespread criticism over its use of armed, masked agents to conduct raids in public places against undocumented migrants.

Trump blamed the Dallas attack on rhetoric directed at ICE by "Radical Left Democrats."

After ICE immigration raids in Los Angeles spurred unrest and protests earlier this year, Trump dispatched the National Guard and US Marines to the California city.