Two detainees have died and another is critically injured after a suspected sniper opened fire at an immigration detention centre in Dallas, Texas, officials say.

Authorities said the gunman died from a self-inflicted gunshot wound. No law enforcement personnel were injured.

FBI Director Kash Patel posted a photo on X of unused ammunition recovered from the scene. One casing has the phrase "ANTI-ICE" on it.

The FBI is investigating the incident as "an act of targeted violence". The shooting follows a string of attacks on Immigration and Customs Enforcement (ICE) facilities across the US in recent months.

"While the investigation is ongoing, an initial review of the evidence shows an ideological motive behind this attack," FBI Director Patel wrote on X.

"These despicable, politically motivated attacks against law enforcement are not a one-off."

Dallas police said a preliminary investigation determined the suspect had opened fire from an adjacent building.

The FBI is investigating the incident as "an act of targeted violence", special agent Joe Rothrock said.

He said early evidence suggested rounds found near the gunman contained "messages that are anti-ICE in nature".

"This is just the most recent example of this type of attack," he said.

Acting director of the Dallas ICE office Joshua Johnson told the news conference it was the second time he has had to stand in front of the media and talk about a gunman at one of his facilities.

"The takeaway from all of this is that the rhetoric has to stop," he said.

Texas Senator Ted Cruz also spoke at the news conference, condemning "politically motivated violence".

"Your political opponents are not Nazis," he said, urging people not to demonise each other for partisan reasons. "The divisive rhetoric, tragically, has real consequences."

US Vice-President JD Vance posted on X: "The obsessive attack on law enforcement, particularly ICE, must stop. I'm praying for everyone hurt in this attack and for their families."

Republican Governor of Texas Greg Abbott took to X to describe the shooting as an "assassination" and said it would "NOT slow our arrest, detention, & deportation of illegal immigrants".

The ICE field office in Dallas has been targeted by a series of protests this summer.

A man was arrested in August after he entered the facility claiming to have a bomb in his backpack, according to the DHS.

The 36-year-old US citizen, Bratton Dean Wilkinson, had shown the building's security staff a device on his wrist that he described as a bomb "detonator," the DHS said.

"These incidents come after months of smears and rhetoric by activists, politicians, and the media comparing ICE law enforcement to the Nazi Gestapo, kidnappers, and the Secret Police," a senior official said at the time.

Last month shots were fired at ICE offices in San Antonio, Texas. No injuries were reported in that incident, which ICE blamed on "political rhetoric".

Another shooting occurred on the 4 July public holiday at an ICE facility in Alvarado, Texas, after a protest escalated into a face-off with police. An officer was shot in the neck, and survived.