(CNN) A Florida teenager who allegedly told police he stabbed three people, one fatally, after he was mocked for being a Muslim was already on the FBI's radar and about to be arrested on federal charges, police reports show.

The FBI, which met with suspect Corey Johnson, 17, of Jupiter last year, began monitoring his social media accounts because of terror-related activity -- almost a year before Monday's bloody knife attack on two 13-year-old boys and the mother of one of the boys at a birthday sleepover, according to a police report.

The investigation -- which stretched back to at least January 2017 -- involves Johnson's alleged interest in ISIS, social media activity that included watching beheading videos, a threat Johnson allegedly made against a school in northern England and his reported fascination with dictators and extremist groups such as the Ku Klux Klan, according to reports compiled by Jupiter police.

Palm Beach Gardens investigators said Johnson's faith led to Monday's attacks, which left the boy celebrating his birthday dead.

Investigators say in a probable cause affidavit that the suspect confessed that he stabbed his victims because one made fun of the way he prayed and kissed the ground and because another treated celebrities as if they were gods, something Johnson said Muslims don't condone.

"This was not a random act of violence," Palm Beach Gardens interim police Chief Clint Shannon said Tuesday. "All persons involved in this incident knew each other."

Johnson will face charges of first-degree murder and attempted murder, Shannon said. CNN called the Palm Beach County State Attorney's Office about Johnson's legal status but the office said "nothing is available for public disclosure."

It was unclear whether Johnson had an attorney.

A woman at the Johnson home told CNN on Wednesday: "We don't have anything to say. Our hearts break for everyone of course, but we just don't have the words."

Investigation began 14 months ago

Shannon said Johnson's alleged "violent tendencies" were the subject of a joint investigation, which included the FBI, Jupiter police and other local agencies.

According to Jupiter police reports, an FBI agent told a Jupiter investigator on March 5 that federal authorities were preparing arrest affidavits for Johnson and that would come "in the next several weeks."

The police reports say in January 2017 a Palm Beach County sheriff's detective contacted Jupiter police about Johnson, saying he had reached out to ISIS on the Internet and wanted to join the terrorist organization.

The detective said Johnson had allegedly been watching ISIS videos online, including beheadings. In March 2017, Johnson denied affiliation with ISIS, police said.

The Palm Beach County Sheriff's Office had gotten involved when the suspect's sister reported him to school resource officers, telling them he had hit her in the fall of 2016. She also told school officers that her brother was a white supremacist who liked Hitler and supported Oklahoma City bomber Timothy McVeigh, who killed 168 people in 1995.

On January 12, 2017, authorities from the FBI, Jupiter police, Palm Beach County schools and the sheriff's office met to discuss Johnson. School police said Johnson had a history of making anti-Semitic and homophobic remarks, according to the Jupiter police report.

After the meeting, the FBI agents told police that they had been contacted by a foreign nation's counterintelligence agency and Johnson had threatened a high school in England. The threats caused 100 students to be removed from school because of fears of a possible attack.

Jupiter police interviewed Johnson's mother and grandmother, the report says, on January 19, 2017. The grandparents told them Johnson had recently discovered religion and had been studying the Quran.

The investigator relayed the information to the FBI and was told the federal agency was monitoring his social media accounts.

Teen denied ISIS affiliation

At some point in the investigation, an FBI agent said the agency didn't want to pursue criminal charges and thought a "redirection approach" to the juvenile's behavior would be best, the Jupiter police report says.

The FBI met with Johnson on March 29, 2017, and received his permission to mirror his computer so agents could see what he was looking at on the Internet.

Johnson denied an affiliation to ISIS but said he was supportive of militant cleric Anwar al-Awlaki, who was killed by a US drone missile in Yemen in 2011, the report from the investigation says.

The agents told him to "cease all social media activities related to ISIS and any other terrorist organizations," but the FBI said he did not do so and during the summer of 2017 they began to consider bringing charges against the teen, according to the documents.

Authorities met in late February to talk about possible charges, but the assistant US attorney wanted more information from Johnson's social media accounts, the report says.

About two weeks later, the FBI told Jupiter police it was working on the documents needed to arrest Johnson, according to the report. It is unclear what charges Johnson would have faced.

CNN called the FBI office in Miami on Wednesday but didn't receive an immediate response.

Suspect took knife to sleepover

Shannon, the Palm Beach Gardens police chief, said Johnson told them he bought the knife Sunday and took it to the Palm Beach Gardens house where Elaine Simon lived with her two sons. Johnson and Jovanni Brand, who was turning 13, had been invited to spend the night.

According to police, Johnson told them Brand referred to famous people as gods and the other victim, Simon's 13-year-old son, made fun of his faith.

Police say that Johnson used the knife to cut Brand's throat as he slept then stabbed Simon 12 times as she came up the steps to investigate the noise in the house. The 13-year-old son came to her aid and was stabbed 32 times, police said.

Mother and son are both in a hospital. The chief didn't give their conditions.

Investigators say that Johnson said he was "reading the Quran from his phone just prior to the attack to give him courage to carry out his intentions."

The other son, who is 15, was not attacked, police said. According to police, the boy said he often had watched violent jihadist videos with Johnson and viewed some on the night of the sleepover.

Johnson barricaded himself in a bedroom and was arrested by a SWAT team.