A shooter with a rifle opened fire from a nearby roof onto a U.S. Immigration and Customs Enforcement location in Dallas on Wednesday, killing at least one detainee and wounding others before taking his own life, authorities said.

The exact motivation of the attack was not immediately known. The FBI said at a morning news conference that ammunition found at the scene contained anti-ICE messaging. The head of the agency, Kash Patel, released a photo on social media that shows a bullet containing the words “ANTI-ICE” written in what appears to be marker.“

The shooter fired indiscriminately at the ICE building, including at a van in the sallyport where the victims were shot,” the Department of Homeland Security said in a release.

The detainees who survived are in critical condition at a hospital, DHS said.

Homeland Security spokesperson Tricia McLaughlin told Fox News that no ICE agents were injured.

According to McLaughlin, authorities believe the shooter was shooting at law enforcement and detainees from a nearby building.

A law enforcement official identified the suspect as 29-year-old Joshua Jahn.

The official could not publicly disclose details of the investigation and spoke to AP on condition of anonymity.