The European Court of Justice (ECJ) ruled Tuesday that national governments may not permit the “general and indiscriminate retention” of traffic and location data of electronic communications.

The court’s ruling comes from a long procedural history. In 2015, an Irish court convicted Graham Dwyer of the 2012 murder of Elaine O’Hara. Dwyer appealed to the Irish Court of Appeal and argued that traffic and location data of his phone calls were improperly admitted as evidence at trial. Dwyer separately initiated a civil action in Ireland’s High Court challenging a 2011 Irish data law on the basis that it violated EU law. The High Court ruled for Dwyer, and prosecutors appealed to the Irish Supreme Court. The Irish Supreme Court then referred the question to the ECJ.

The ECJ found that settled EU law “enshrines” a guiding principle prohibiting retention of traffic and location data of electronic communications. This principle is part of the fundamental right of EU citizens to “the respect for private life and the protection of personal data.”

Members of the court acknowledged that, based on prior case law, member state legislatures may allow limited data storage in specific cases for “the purposes of combating serious crime and preventing serious threats to public security.” These narrow exceptions include:

the targeted retention of traffic and location data which is limited, according to the categories of persons concerned or using a geographical criterion; the general and indiscriminate retention of IP addresses assigned to the source of an internet connection; the general and indiscriminate retention of data relating to the civil identity of users of electronic communications systems; and the expedited retention (quick freeze) of traffic and location data in the possession of those service providers.

The exact effect of the court’s ruling on Dwyer’s case is unclear. In a statement, Irish Minister for Justice Helen McEntee said, “[t]he case will now revert to the Supreme Court and the Department of Justice will consider, together with the Attorney General’s Office, the judgment of the Supreme Court when the case is finalised.”