They did not have a judicial warrant. An administrative warrant does not mean you can enter a private place without consent. Let me be clear, it appears that the person in question, Eduardo Flores-Luiz, was due for a pre-trial hearing that morning, the families of the victims were in the courtroom. From what I understand, it appears he did not have the hearing. He was facing serious charges. I have not found anything that suggests to me that he was arrested or in jail prior to this, unless somebody has proof to say otherwise. If that is the case, then that is the fault of the local/state police, because I find it very hard for me to believe that for someone facing serious charges would not be arrested and in jail, unless he was arrested and then he posted bail, which from what I understand, it didn't appear so.
Also, if they were going to arrest Flores-Luiz, why didn't they tell whoever runs the courtroom building in advance? What I mean is, why didn't they tell the chief judge or whoever does the rounds in the courtroom in advance? If the affidavit is true, and that his charges were of public record, are you telling me that the FBI/ICE didn't bother to find out when his hearing was due? Sounds to me like ICE/FBI is mad that they were embarrassed because they thought an administrative warrant meant they could just go a courtroom and arrest somebody without a judicial warrant. I have never been in a courtroom, so I can't speak for myself, but I would imagine that judges, especially these days, have a lot of people they have to see in the docket over the course of a day, never mind in a week. But again, I am skeptical of believing anything this corrupt FBI/ICE are saying.