Diane Sterling was shot and killed by Sergei Mishnev.
Sergei Mishnev arranged a rooftop meeting between Gibbs and Diane in Episode 12, "Check." Mishnev was watching the meeting through a sniper rifle's scope and then took his shot at Diane. She was killed instantly with the circumstances of the murder matching those of NCIS Special Agent Caitlin Todd, whose murder occurred ten years earlier.
Watch NCIS Tuesdays at 8/7c.