First, the teenager's 2-year-old sister, Desiree, stopped breathing while he was giving her a bath, authorities said.

Less than three months later, investigators said, his 11-month-old brother was discovered unresponsive after he tucked the infant in and put him to bed in their home in eastern Indiana. Both of the young children died.

Prosecutors said the 14-year-old, who is not being named by The Washington Post because he is a juvenile, has been charged with murder in the deaths of his half-sister, Desiree McCartney, and his step-brother, Nathaniel Ritz. Prosecutors said the teen told police he smothered them because he "had to set them free from this hell," according to a probable cause affidavit.

But the 14-year-old's mother, Christina McCartney, told Fox affiliate WXIX that her teenage son is not "the monster that people are trying to portray him as," explaining that her son has since said that he and his younger siblings allegedly had been abused by his stepfather. "It's a mother's worst nightmare to have such a precious family and then to wake up to the worst possible outcomes."

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It's unclear who is representing the teen in the case.

One Monday evening in May 2017, McCartney arrived at home in Osgood, Indiana, about 40 miles from the Ohio border, and learned that something was wrong with her daughter Desiree.

McCartney later told authorities that when she walked in the house, she saw her teenage son holding Desiree, who was wrapped in a towel, and the teen told her that he could not get the toddler to respond, according to court documents. The documents state that McCartney "knelt down and shook Desiree and she still didn't respond" so she called 911 and performed CPR until help arrived.

The boy, who was 13 at the time, told police that he was helping to bathe his little sister, who had been vomiting in the bathtub. At one point, he said, he went to get some baby wipes and, when he returned, he found the toddler "floating in the water on her belly," according to the court documents.

Five days later, Desiree was pronounced dead. The initial autopsy found that the brain had been deprived of oxygen for a prolonged period of time, but the cause of death was undetermined.

That July, it seemed to be happening all over again.

McCartney told police that she had told her son to put his baby brother to bed just before midnight on July 20, 2017. Minutes later, she said, her son told her that "something is not right with Nathaniel, he is not acting right," according to court documents.

The teenager initially told police that shortly after he put the 11-month-old to bed, he noticed that "Nathaniel was not moving, so he went and checked on him and he was cold and looked bright white," the court documents state. He told police he took the baby into his mother's and stepfather's bedroom and the couple started performing CPR.

Prosecutors said the baby's heart had stopped beating.

Again, the family called 911.

Again, the paramedics came.

Again, a young child mysteriously and tragically died.

And for the second time, the coroner was unable to determine a cause of death, according to the court documents.

It wasn't until several months later that things started to come into focus. In September of last year, the teenager's mother contacted investigators, telling them the boy had mutilated a couple of kittens - one with a puncture wound to the head and its "insides" hanging out of it and another with a puncture wound to its mouth and head, according to the court documents. It's unclear whether investigators ever spoke with him about the animals, but they asked him again about his siblings.

When investigators went to question the boy in December, he talked about Lazarus - the biblical figure said to have risen from the dead. He told investigators he had been having dreams about his siblings. He talked about saving them "from hell and the chains of fire" and said he "had help from an angel to free them," the documents state.

During the conversation, according to the court documents, authorities said the teen admitted that while bathing his little sister last year, he "put the towel over her head" and that, while putting his brother to bed, he "put the blanket on Nathaniel's head." He told the investigators that he "didn't plan for these things to happen" and he "didn't want to hurt them" but that he had to "set them free from this hell," the documents state.

When the investigators asked the teen to define his "hell," he replied, "chores," according to the court documents.

Following the babies' deaths, McCartney, the teen's mother, told WXIX that her son had witnessed his stepfather "being mean to the babies." She said the stepfather "pushed them down on purpose. He would lock them up in the bedroom to try to shut them up. He painted a pretty bad picture. As a mom that's hard to swallow that was going on and I didn't know."

Prosecutors said McCartney was Desiree's mother but was not related to Nathaniel; Nathaniel belonged to the stepfather. The nature of the relationship between McCartney and the stepfather is also unclear. Court documents state that he was McCartney's "fiance," but local news reports call him a "boyfriend."

McCartney told WXIX she believes her teenage son committed the crimes "under threat" - that he may have been forced to do it by his stepfather. The stepfather could not immediately be located for comment.

Following the investigation, the autopsies were amended to show the children died of asphyxia because of smothering, and the coroner ruled that the manner of the deaths was homicide.

Prosecutors said the teen, who is now being held in a juvenile detention center, is set to undergo an evaluation to determine whether he is competent to stand trial, according to the TV station.