Notes written by the person suspected of opening fire on an immigration facility in Texas indicate he was targeting ICE agents and did not intend to harm detainees, acting US attorney for the Northern district of Texas Nancy Larson said.

One detainee was killed and two others critically injured after a suspected sniper opened fire at an Immigration and Customs Enforcement (ICE) centre in Dallas on Wednesday, officials said.

Speaking at a news conference on Thursday, Ms Larson identified the shooter as 29-year-old Joshua Jahn, adding that notes he had written were found at his home.

She stated Jahn intended to maximise lethality against ICE personnel and to maximise property damage at the facility.

He hoped to minimise any collateral damage or injury to the detainees and any other innocent people, she added. It is clear from these notes that he was targeting ICE agents and ICE personnel. The victims have not yet been identified.

FBI director Kash Patel indicated that evidence points to a high degree of pre-attack planning. One of the handwritten notes found stated, Hopefully this will give ICE agents real terror, to think, 'Is there a sniper with AP rounds on that roof?' Larson indicated that the shooter likely acted alone.

That morning gunshots sprayed the length of the building, hitting windows and law enforcement vans in the sallyport area, she noted, adding that detainees were also hit. The attack was described as a calculated, ambush-style assault targeting ICE personnel.

Marcos Charles, ICE field office director of enforcement and removal operations, condemned the violent rhetoric against ICE, saying, In contrast to those who would demonize our men and women, yesterday our brave officers ran back into danger to save the detainees while shots were still being fired.