Aerial footage shows the Dallas immigration facility and police response where U.S. officials say two detainees were fatally shot and and third person was critically injured.

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 detainee who survived was in critical condition at a hospital, DHS said.