TULSA, Oklahoma (AP) — An Oklahoma 16-year-old will be charged as an adult alongside his older brother in the stabbing deaths of his parents and three siblings, authorities said Saturday.

The suspect will face the same counts as his 18-year-old brother, Robert Bever, who is accused in a booking document of five counts of first-degree murder and a count of aggravated assault in the Wednesday night attack.

Police have not released the 16-year-old's name, but Tulsa County District Attorney Steve Kunzweiler confirmed Saturday both siblings have been booked on first-degree murder complaints. It wasn't immediately clear in jail records when either sibling is due in court.

Trial as an adult means that the 16-year-old could get a maximum sentence of life in prison or life in prison without parole if he's convicted of first-degree murder, Kunzweiler said. While the death penalty is legal in Oklahoma, the U.S. Supreme Court in 2005 banned the execution of anyone under the age of 18 at the time a crime was committed.

The brothers are accused of fatally stabbing their parents, two brothers and a sister.

An unidentified 13-year-old sister was in serious but stable condition in a hospital, and an unnamed 2-year-old sister wasn't harmed and was placed in state custody.

Detectives continued to search for a motive, scouring social media sites believed to belong to Robert Bever in hopes of finding clues. Cpl. Leon Calhoun, a spokesman for the department, said both siblings were cooperating with investigators.