When Herod heard of Jesus’ miracles he said “John, whom I beheaded, has been raised” (Mark 6:16). Then Mark recounts the story of how John had died, in order to explain Herod’s statement. The next verse begins
For it was Herod who had sent and seized John and bound him in prison for the sake of Herodias, his brother Philip’s wife, because he had married her. (Mark 6:17)
Note the shift back in time implied by “had sent”. The next few verses give the details of how John came to be killed.