A Florida teenager -- who had previously been investigated for "alleged violent tendencies" -- watched jihadist videos and read the Koran "to give him courage" before stabbing a 13-year-old boy to death and seriously injuring two others at a birthday sleepover Monday, cops said.

Corey Johnson, 17, was charged with first-degree murder and attempted first-degree murder after police said he confessed to the assault at the 13th birthday sleepover party at a BallenIsles Country Club home, the Palm Beach Post reported.

Jupiter Police, Palm Beach County School District police and the FBI previously received “intelligence gathering” on Johnson that prompted an investigation into his “alleged violent tendencies,” the Palm Beach Post reported.

"Corey Johnson is out there with a knife. He's a murderer. That's all I know." — Neighbor

Johnson reportedly told police he woke up at 4 a.m. Monday at Kyle Bancroft’s home and had an urge to kill Kyle’s mother, Elaine Simon; Kyle’s brother, 13-year-old Dane Bancroft; and Dane’s friend, 13-year-old Jovanni Sierra. The three victims were each asleep at the time, the Post reported.

"In his statement, Johnson advised he stabbed the victims because of his Muslim faith," the affidavit stated.

Johnson and Kyle Bancroft allegedly watched violent jihadist videos the night before the 17-year-old carried out the deadly stabbing. Using a knife he brought to the home, Johnson stabbed Sierra multiple times before slitting his throat.

Simon, apparently hearing Sierra moaning in pain, began walking upstairs around 6 a.m. when Johnson allegedly lunged at her. Police say he stabbed her at least a dozen times before she fell down the stairs. Dane was stabbed 32 times while attempting to help his mother, according to the Palm Beach Post.

“He’s a hero," the boy's father said during a phone interview with the Post. “He saved his mom."

Sierra was pronounced dead at the scene. Simon and Dane were taken to the hospital, where Simon remains in good condition as of Tuesday.

Kyle Bancroft and Johnson were not harmed in the incident.

Police were alerted after Simon ran to a neighbor begging to call 911.

"She rang my bell, she’s bleeding," the neighbor said later. "Corey Johnson is out there with a knife. He’s a murderer. That’s all I know. They're trying to help the kid the kids filled with blood."

Johnson was taken into custody about 8 a.m. after briefly barricading himself in a room.

He later told authorities he was upset Sierra, who turned 13 on Monday, called celebrities “gods” because it went against his Muslim faith. Johnson also said Dane made fun of his Islamic faith.

The teen told police he read the Koran before going to the sleepover "to give him courage to carry out his intentions,” according to an affidavit.

The Palm Beach County Sheriff’s Office received information prior to the stabbings that Johnson reached out to ISIS online and expressed his desire to join the terror group.

Law enforcement officials at the time investigated Johnson by tracking his social media accounts and movements and interviewing his family. The teen was also being investigated for several threats that prompted evacuations at a British school in October 2016.

Authorities had planned to file charges against Johnson for international school threats a week before the sleepover rampage, according to the report.

Johnson went to William T. Dwyer High School in Palm Beach Gardens, but recently withdrew, according to the report. Dane Bancroft and Jovanni Sierra were attending Watson B. Duncan Middle School.