Does the DA decide in Wisconsin or does it have to go to a grand jury?
I'm 90% they require GJ's.
I was digging in Wisconsin State Law, and a DA can file a complaint without a Grand Jury. The complaint is filed with a judge, who decides whether there is probable cause for a summons/warrant. This is the beginning of the filing of charges. Another option is to convene a Grand Jury, who can decide or not, but the GJ is not required. See section 968. et all, and specifically, 968.06.
