Recent events in the media (the recent horrible gun attack on Congresswoman Giffords, a federal judge and others by Jared Loughner in Tucson, AZ) remind us in the criminal justice legal community to revisit those legal events that trigger concurrent jurisdiction of both the federal authorities and the state (or local) authorities. There are somewhat limited times when the federal authorities can get involved in violent crimes that happen in a small geographic area. Usually, the alleged crime must involve "interstate commerce," that is, the geography of the crime must extend across state lines (e.g. kidnapping across state lines, interstate organized crime, or interstate/international drug trafficking) or involve federal officials or a specifically stated type of crime (e.g. internet child pornography [violates both Fed and local law], threatening, inuring or killing a Federal public official, bank robbery, skyjacking, using U.S. Mails for any criminal purpose, and counterfeiting). Federal prosecutors can often invoke this "interstate commerce" requirement by simply proving that a defendant used the mails or national electronic means (phone or email) just once during the course of the conduct which gives rise to the crime. Sometimes conduct can be simultaneously prosecuted by both the federal authorities and state authorities - but most times the federal and state officials simply agree on who will prosecute what portion of the crime. Further, often federal and state authorities put together joint task forces to work on specific areas of criminal activity.
Recent events in Arizona will involve some delegation of who will prosecute what. Most likely the defendant will be prosecuted federally for his tragic actions against the Congresswoman, her staff and the federal judge, while the tragic actions involved against the other victims will involve state prosecution - although nothing is certain at this time. The federal authorities have already filed preliminary federal charges.