Video exclusively obtained by ABC News shows the moment gunfire erupted outside the Dallas Immigration and Customs Enforcement field office on Wednesday, killing one detainee and critically injuring two others.

The surveillance video shows officers and detainees ducking for cover near two vans in the facility’s sallyport. Authorities said a sniper fired toward the building from a nearby rooftop in the early morning attack.

Detainees were being unloaded from a van when the shooting occurred and were being restrained for transport, per proper procedure, officials said. There were 10 detainees in the van, coming from a local jail, an ICE official told ABC News.

“Heroic” officers worked to take the detainees to safety while under fire, Nancy Larson, acting U.S. attorney for the Northern District of Texas, said at a news conference on Thursday.

In the video, armed agents could be seen carrying victims away.

In footage from inside the building, the detainees could be seen running while handcuffed and their legs shackled, some falling to the ground amid the rush to get away from the gunfire.

Authorities said the suspected shooter — 29-year-old Joshua Jahn — “fired indiscriminately” at the ICE building and van with a bolt-action rifle. He allegedly wanted to “maximize lethality against ICE personnel and maximize property damage at the facility,” Larson said.

He allegedly left behind a note that said, “Hopefully this will give ICE agents real terror, to think, ‘is there a sniper with AP rounds on that roof?'” FBI Director Kash Patel said in a statement, the note referring to armor-piercing bullets.

Jahn, who died from a self-inflicted gunshot wound, “indicated he did not expect to survive the attack” in his handwritten notes, officials said Thursday.

Investigators believe Jahn acted alone and have not found that he was a member of any specific group, Larson said.

No government agency was mentioned in his notes other than ICE, but he did express a “hatred for the federal government,” Larson said.

Patel said investigators have obtained evidence indicating “a high degree of pre-attack planning,” with Jahn’s alleged activity including searches last month on apps that track ICE agents and downloading a document called “Dallas County Office of Homeland Security & Emergency Management,” which has a list of DHS facilities.

Acting ICE Director Todd Lyons said he would put all ICE facilities on a higher alert in the wake of the shooting.