There are a lot of Amentan countries. Vanda Nossëo representatives are dispatched to all of them. These Elves (two with black hair, one with silver) take a shuttle down from the lightleaper to a country called Calado, and radio ahead to request permission to land at a elegant modern spaceport.
Vanda Nossëo sends Rivik a courtesy notification that there's an investigation into loss of life in connection with interdimensional contact and that galactic observers will be present for the foreseeable future, as will the media. Should Rivik choose to cooperate with the criminal investigation into the murders, the observers are equipped to collaborate with them and provide forensic support and factfinding; should Rivik find that events do not merit prosecution under local law, Vanda Nossëo will handle it all themselves.
Vanda Nossëo has made the following arrests for murder in connection with the ongoing investigation. They understand that extended contact can cause all kinds of problems and will keep the observers and investigators as unobtrusive as possible; most Rivikni citizens should never even see one, and they'll wrap up the investigation as quickly and neatly as they can.
Galactic law allows for prosecutions for rape, torture, murder, enslavement, and arbitrary deportation or forced transfer of populations. The laws are straightforward and there's a translation available in every Amentan language. Vanda Nossëo will enable local prosecutions if local governments are willing and able to prosecute. The case law and some relevant helpfully-chosen examples are also available in every Amentan language.
Examples are mostly people who killed members of oppressed ethnic or social or religious groups after learning that the aliens planned to give them rights. There is a lot of case law. This comes up a fair bit.