Green darner (m)eating 8 spotted skimmer(f)

Large blue dragonfly with what I thought was a robber fly clutched in its mandibles. The pair tumbled to a landing on the sand dune and I snapped the pictures.Then they flew away. When I got close enough to snap another picture, it dropped the robberfly and flew away. Now that I can see the photo more clearly, I see the robberfly was another dragonfly about half the size of the first