II Melechim 12:21 and II Divri HaYamim 24:25 both indicate that servants conspired and killed King Joash. There seems to be a little bit of confusion on the murderers' names---Yozacar vs Zabad ben Shimeath(an Ammonite in II Div. HaYamin) and Yehozabad ben Shomer/Shimrit(a Moabite in II Div. Hayamin). Whatever their names and nationalities were, II Melechim 14:5 and D. HaYamim 25:3 both indicate that Amaziah followed Torah law by executing his father's murderers and not their sons.
On the other hand, Pritchard's ANET has an inscription from the ruling Assyrian king, Adad-Nirari III, which states that, in 804/803 BCE, he laid siege to Jerusalem, took the city on 2 Adar, and appointed a new king of his liking(16-year old Amazyahu). Is there any information anywhere in the extant literature that clears up this situation??