What is unusual is that anyone would be so stupid as to commit this crime in a FEDERAL BUILDING.
Of course the US Marshalls would have made the arrest. It was a Federal Crime on Federal Property. The state has no jurisdiction on this one.
I understand the part about this being a federal crime and that a federal agency would be making the arrest. I just thought that the Federal Marshals had other duties. Perhaps they just had offices in that building & they were the closest people "with a badge?" The FBI has them now, and I expect that this will bring heavy charges.