Items found at the scene of the attack at the U.S. Immigration and Customs Enforcement (ICE) office in Dallas were engraved with the phrase “ANTI-ICE,” the FBI reported on Wednesday.

Among other things, a bullet casing was engraved with “ANTI-ICE,” said FBI Director Chris Wray.

The case is being investigated by the federal office as an act of “targeted violence,” an FBI agent said, adding that initial evidence indicates the incident was “anti-ICE in nature.”

During the bloody incident early Wednesday morning (local time), four people were injured by gunfire, and one lost their life. The perpetrator shot himself and also died.

Initial findings indicate there is an “ideological motive behind this attack.”

The shooter was on the roof of a nearby building, according to local TV station WFAA, citing sources familiar with the matter. “He was likely a sniper,” ICE Deputy Director Todd Lyons told CNN.

The shooting victims were being transported to the building for processing and repatriation, Fox News reported, citing Dallas police sources.

The perpetrator fired multiple times from a nearby roof or elevated position near the secure entrance of the local office, an ICE spokesperson said.

It is not yet clear who the suspect’s target was. The shooter fired only at detainees, but authorities remain uncertain whether he specifically targeted those detainees.