Herod himself had ordered John's arrest, and he had him tied up and put in prison. Herod did this because of Herodias, whom he had married, even though she was the wife of his brother Philip. 1
References for Mark 6:17
6.17, 18 Lk 3.19, 20.
Scripture taken from the Good News Translation - Second Edition, Copyright 1992 by American Bible Society. Used by Permission.