U.S. District Judge Lucy Koh voiced concerns about allowing legions of email users to band together with claims that Google Inc. violated their privacy by automatically scanning Gmail messages.

At a hearing in the U.S. District Court for the Northern District of California on Thursday, Koh grilled plaintiffs lawyers on issues ranging from how they will determine who belongs in the proposed classes to how they will demonstrate what email users collectively knew about Google’s policies. Google insists that email scanning is common knowledge. But plaintiffs claim Google has not publicly explained how it intercepts messages to help sell ads, and they accuse the company of violating the Electronic Communication Privacy Act and a handful of state privacy laws.