Wisconsin doesn't have a death penalty.
The feds DO have a death penalty - especially if you lay hands on one of theirs.
If the perp doesn't get fried but pronto, the social contract with the feds is busticated, and federal workers are on their own.
I believe the physical location of where a crime is committed dictates jurisdiction. I.e., Territorial jurisdiction. In this case, I would assume that the city of Milwaukee or state of Wisconsin will have jurisdiction for trying the perp(s).
That is pretty much the case by now. Only the higher echelon of managers and executives get any real consideration. Of course, the law enforcement types (FBI, ATF, US Marshals, etc.) are part of "The Family". Perps who mess with them will be taken out. "Resisting Arrest" or some such thing. No need for the aggravation of a trial.
Everybody else is just collateral damage if they fall prey to criminals.
I am unsure how USPS employees are ranked in the scale of values. A worthwhile indicator will be to check back later to see if they "round up the usual suspects", and actually prosecute someone.
Meanwhile, I have prayers for the family and friends of the deceased.