Newly released court documents show that a teenager in a Phoenix suburb threw his 3-year-old brother down a flight of stairs before fatally stabbing him multiple times with a carving knife.

Investigators say the attack came without warning or provocation and the teen also is accused of wounding his 9-year-old sister as she tried to stop the assault Saturday night in Youngtown.

The 16-year-old boy was transferred Tuesday from a juvenile detention center to a downtown Phoenix jail.

He's being held on suspicion of second-degree murder and aggravated assault and authorities plan to prosecute him as an adult.

The Associated Press is not identifying the teen because he's a juvenile.

The teen's bond was set at $1 million at his initial court appearance Wednesday. He doesn't have a lawyer yet.