“She said somebody attacked her, but you have to get somebody here quick!” the neighbor relayed to a 911 operator, according to a recording of the emergency call. “She’s delirious. [The attacker] had a knife. … She’s dying over here. The kids are out here bleeding!”

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In the audio, the woman can be heard in the background calling out a name: Corey Johnson. It was a boy her older son had known for more than 10 years and invited to their home for a sleepover the night before, she told police later. And he was the one who had attacked them overnight, she said.

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Johnson, 17, later told police that, as everyone else in the house was sleeping, he decided to go on a stabbing rampage, killing one and seriously injuring two with a knife he had bought the day before.

The Washington Post does not typically identify juveniles charged in criminal cases. However, on Thursday, a grand jury in Florida indicted Johnson on adult felony charges of first-degree murder and attempted first-degree murder.

The stabbings, which occurred March 12, have raised questions about Johnson’s long history of violent tendencies and extremist beliefs. As early as middle school, school officials and local law enforcement had noted that Johnson had a fascination for Nazis and white supremacists, had reportedly antagonized female classmates, and had made derogatory comments against Jewish and LGBT people.

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More than a year ago, Johnson’s behavior became so disturbing that authorities reported him to the FBI, concerned that he was communicating with the Islamic State and other terrorist organizations, according to police records. For months, agents surveilled his online activity and, as recently as early March, appeared ready to arrest him on unspecified federal charges.

That arrest would never come. But another one, in connection with a bloodier crime, would.

After the sleepover attack, Johnson was arrested and detained at the Palm Beach County Juvenile Detention Center. He told investigators that he had carried out the stabbings “because of his Muslim faith,” according to a probable-cause affidavit from the Palm Beach Gardens Police Department.

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The night before, four teenage boys — Simons’s 15- and 13-year-old sons, Kyle and Dane Bancroft; Dane’s friend, 13-year-old Jovanni Sierra; and Johnson, a friend of Kyle — were settling in to spend the night at Simons’s home in BallenIsles Country Club, a gated community in Palm Beach Gardens, police said.

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At some point in the day, Johnson and Kyle had watched a video on a cellphone about Islamist militants, the affidavit stated. Kyle told police that he didn’t believe in Islam but that Johnson did — and that the two would “often” watch videos that were “extreme and encourage death to nonbelievers,” the affidavit stated.

At the sleepover, Johnson claimed the younger boys had offended him with regard to his “religious beliefs,” police said.

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“Jovanni was [idolizing] famous people and referencing them as gods (which went against the Muslim faith) and Johnson felt Dane ‘made fun of’ his Muslim faith and the fact that Johnson prayed and kissed the ground,” the affidavit stated. “Johnson advised he watched violent videos on his phone and computers … and was reading [the Koran] from his phone just prior to the attack to give him courage to carry out his intentions.”

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Johnson later told police that he decided about 4 a.m. Monday that he would kill Jovanni, Dane and Dane’s mother before they woke up, the affidavit stated. Johnson first went to a loft area where Jovanni was sleeping and fatally stabbed him and cut his throat, police said.

About 5:45 a.m., Simon — awakened by commotion and the sounds of Jovanni moaning — went upstairs to check on her sons and their two friends, police said. Johnson attacked her next, stabbing her so many times that she fell down the stairs; Johnson also stabbed Dane 32 times, police said, reportedly as he tried to protect his mother.

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When officers responded, they found Simon and Dane severely injured.

Jovanni was pronounced dead at the scene; it was the morning of his 13th birthday.

Kyle was unharmed, police said.

Police records show that Johnson’s past behavior and ideologies were so troubling that the FBI had kept him on its radar since 2016. Meanwhile, local law enforcement agencies had known about Johnson far longer than that.

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As early as 2014, when Johnson was a student at Independence Middle School, a classmate reported him to school police for allegedly stalking her on social media and asking her for sex, the Palm Beach Post reported. When Johnson was in seventh and eighth grades, he had made “anti-Semitic, anti-homosexual statements” and had previously pledged he had similar beliefs as the Ku Klux Klan, according to records obtained from the Jupiter Police Department.

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In December 2016, when Johnson was a 10th-grade student at Dwyer High School, his younger sister reported that he had punched her in the head but that “her mom blew it off,” police records stated. The sister also told police that she was concerned about her brother’s “anger and bullying issues,” his lack of friends, his fascination with Nazi leader Adolf Hitler, his white supremacist beliefs and his negative talk about people, especially girls.

At the time, police said they searched Johnson’s Facebook page and found that his profile photo was that of a Nazi swastika. With enough information to be concerned, Jupiter police contacted the FBI’s joint terrorism task force.

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At a meeting about Johnson in January 2017, FBI agents reported that they had linked Johnson’s computer at his home in Jupiter, Fla., to threats that had been made through social media against McAuley Catholic High School in the United Kingdom, police records said.

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The threats were “so severe in nature that up to 100 students were removed from the school fearing some kind of attack,” police documents stated.

In an interview that month with Johnson’s mother and grandmother, police learned that the teenager had been fascinated with dictators like Hitler, Joseph Stalin and Kim Jong Un. Johnson had also reportedly begun studying the Islamic holy book and obtained a Muslim prayer cloth and a kufi, a traditional skull cap. (Local Islamic groups told the Palm Beach Post that Johnson was not known to them.) Johnson’s relatives also told authorities that there were several firearms at their home but that they were kept in a locked safe in the master bedroom, records stated.

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A police detective interviewed Johnson and conducted a brief mental health assessment, and another agency said it would continue to monitor the teen at school. Despite the red flags, FBI agents said they did not recommend pursuing criminal charges against Johnson “as he is a juvenile and [they] believed a redirection approach would be the most beneficial regarding his conduct,” police documents stated.

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A representative for the FBI’s office in Miami did not respond to requests for comment.

Federal law enforcement agents obtained consent to monitor Johnson’s online activity and continued doing so for the next year. In March 2017, FBI agents interviewed Johnson, who “adamantly denied he had any affiliation” with the Islamic State but stated he was supportive of Anwar al-Awlaki, a known terrorist, police records showed. The FBI reportedly told Johnson to cease all social media activity related to the militant group, according to police records.

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In February, federal and local authorities met again and — though it is not specified what additional threats had emerged — determined there was now probable cause for Johnson to be arrested on federal charges. On March 5, just days before the fatal stabbing in Palm Beach Gardens, an FBI agent said he was working with the assistant U.S. attorney’s office on affidavits for Johnson’s arrest and was told they would be “coming in the next several weeks.”

Less than a week later, on March 11, Johnson spotted Jovanni at a pizzeria in Palm Beach Gardens, the Palm Beach Post reported. Jovanni was with his family, celebrating his birthday a day early — and invited Johnson and his group to sit with them, a photo of the smiling group showed.

Jovanni was planning to spend the night at his friend Dane’s house. Johnson would be invited, too, a guest of Dane’s older brother, Kyle.

As they prepared to head to the sleepover, Karen Abreu, Jovanni’s mother, told the newspaper that she urged her son to reconsider so they could continue celebrating his birthday.

“I said, ‘Please come home. It’s your birthday tomorrow. I want to give you a hug and a kiss,’ ” Abreu told the Palm Beach Post. “He told me, ‘Mommy, I love you. I want to hang out with my friends.’ … My last words to my son were, ‘I love you and have fun.’ I never imagined that was the last time I would see him.”