The 14-year-old charged with killing his family of five in north Alabama recently learned the woman he believed to be his mother wasn’t a biological parent, a relative said.

Daisy McCarty, a first cousin of the teen suspect, told WAFF-48 News she believes the revelation led to the Monday night slayings in the tiny north Alabama town of Elkmont in Limestone County. She said the teen had also been acting out in recent months, by burning live animals and breaking into his school, according to the Associated Press.

The teen confessed to fatally shooting his 38-year-old father John Sisk, his 35-year-old stepmother Mary Sisk, and his three siblings, according to the Limestone County sheriff’s office. The three child victims were a 6-month old boy, 5-year-old girl and 6-year-old boy, said Stephen Young, a sheriff’s office spokesman.

McCarty told the TV station her cousin learned last week that Mary Sisk wasn’t his biological mother. The boy’s biological mother died in Indiana in 2011, according to records reviewed by AL.com.

In 2010, the boy’s father asked a judge for full custody of his son, court records show. At the time, John Sisk told the judge that his son hadn’t seen his biological mother since 2008 — when the boy was 3 years old.

When the boy had previously spent time with his mother, the police were called on numerous occassions because she was drunk or on pills, John Sisk wrote in court records. While the boy lived with his father in Alabama, the mother was living in Kentucky at that time, records state. The court never heard from the boy’s biological mother, and John Sisk was granted full custody, records show.

Because of his age, the teen suspect’s name isn’t being released by the authorities or reported by AL.com.

The deadly shootings happened late Monday at the family’s home on Ridge Road in Elkmont.

The gun used in the homicides was in the Sisk home illegally, Stephen Young, a sheriff’s office spokesman, said at a press conference on Tuesday afternoon. Young didn’t take questions and said he couldn’t provide further details. Investigators found the 9-mm handgun during a search Tuesday morning.

What’s next?

Because of his age, the teen is charged with murder in juvenile court, where proceedings are held in secret. His name isn’t being released by the authorities or reported by AL.com. He’s being held in juvenile detention.

If he’s convicted of murder by a juvenile court judge, the teen would face a minimum sentence of a year in a youth detention facility, such as Mt. Meigs. The maximum penalty is detention until his 21st birthday. Information about the sentence or verdict wouldn’t be released to the public.

In Alabama, youths as young as 14 are eligible to face adult charges if prosecutors ask and a judge approves.

A judge can hold a court hearing to determine whether to transfer the case to adult court. The judge would first consider whether prosecutors have enough evidence— or probable cause— to warrant the charges. Next, the judge would consider factors — like the nature of the case, whether the suspect has a juvenile criminal record, the suspect’s mental capacity and success or failure of any prior treatment — to determine whether the teen should be charged as an adult.

In adult court, he could face a capital murder case and jury trial. The maximum capital murder sentence for a juvenile is life in prison without the possibility of parole.

Limestone County District Attorney Brian Jones has said he can’t discuss details of the case because it is currently in juvenile court.