Who killed Ahab's seventy sons?


(He was on a mission from God.)
Ahab had seventy sons in Samaria. And Jehu wrote letters ... to them, saying, ... take ye the heads of the men your master's sons, and come to me to Jezreel by to morrow this time. ...

And it came to pass, when the letter came to them, that they took the king's sons, and slew seventy persons, and put their heads in baskets, and sent him them to Jezreel. 2 Kings 10:1-7