The court was unanimous Thursday in reversing a federal appeals court ruling that sided with the Ontario, Calif., SWAT team officer.
Justice Anthony Kennedy wrote for the court that the officer, Sgt. Jeff Quon, could not assume "that his messages were in all circumstances immune from scrutiny."
But Kennedy said the court purposely avoided a broader ruling about employees' expectations of privacy when using equipment provided by their employers because of rapid and unpredictable changes in technology.
The Ontario department discovered many personal messages, including some that were said to be sexually explicit, when it decided to audit text message usage to see whether SWAT team officers were using their pagers too often for personal reasons.