The leaders of Apple Inc., Google Inc., Microsoft Corp. and other major technology companies on Thursday marked the one-year anniversary of former National Security Agency contractor Edward Snowden’s leaks with a plea to the Senate: Pass legislation to reform government surveillance.

Tim Cook of Apple, Larry Page of Google and Satya Nadella of Microsoft were among nine tech CEOs who signed an open letter to senators, urging them to “ensure that U.S. surveillance efforts are clearly restricted by law, proportionate to the risks, transparent and subject to independent oversight.” The other chief executives on the letter were Tim Armstrong of AOL Inc., Dick Costolo of Twitter Inc., Drew Houston of Dropbox Inc., Marissa Mayer of Yahoo Inc., Jeff Weiner of LinkedIn Corp. and Mark Zuckerberg of Facebook Inc. All of the companies are members of the Reform Government Surveillance coalition, which was formed in December.