Disturbing note written by teen before he allegedly murdered ex-girlfriend following their breakup

A Long Island teen who is alleged to have killed his former high school sweetheart in an attempted murder-suicide left behind a disturbing note detailing the deadline he had set himself, prosecutors have claimed.

Marine recruit Austin Lynch stands accused of killing his 18-year-old ex-girlfriend Emily Finn in a horrifying shotgun attack at his home.

Suffolk County prosecutors claim the attack was in response to the talented ballerina dumping him after going to college.

Authorities claim to have found a truly chilling manifesto on Lynch's phone in the aftermath of the attack, a note which details how he 'f**king hates her,' and reveals how the alleged killer had set himself a dark deadline.

“I have set my mind on leaving this place before my 18th birthday,” the note read.

Assistant District Attorney Dena Rizopoulos said at his arraignment that Lynch had become obsessive after Finn went off to SUNY Oneonta, dumping him just a few weeks before her murder on November 26 - a day before Lynch's birthday.

Finn was killed while returning objects to Lynch's home (Instagram)

Their relationship had broken down after the victim went off to college, with Rizopoulos citing several of her friends had described Lynch as 'possessive, accusatory, and overbearing,' after being left behind in Sayville to join the Marines.

But their three-year high school entanglement ended after an incident on Halloween this year, when Lynch went to see Finn on campus, but the visit ended in fighting and bickering.

Finn would ultimately end their relationship in a phone call, followed by blocking Lynch across various online platforms.

Rizopoulos told the court: “The victim’s friend described the two-week point when the defendant shifted to anger.”

Despite an angry phone call where Lynch called his ex a 'whore' on November 21, Finn decided to get some closure and hand over some personal items at his home.

“The victim came home for the Thanksgiving holiday and made plans to speak with the defendant on Wednesday, Nov. 26, to return some personal items and have a face-to-face conversation for closure,” the prosecutor detailed.

Lynch had enlisted as a Marine after Finn left for college (Instagram)

This friend also corroborated the note that authorities found on the accused's phone, with the attorney adding: “In a conversation the weekend before the murder, this defendant told this friend that he would be dead by Wednesday.”

Despite setting himself this grim birthday deadline, Lynch appeared in court this week to plead not guilty to a second-degree murder charge.

He wore green medical scrubs while his face was obscured by bandages, after a failed suicide attempt in the aftermath of the shooting.

His own parents had rushed into the garage after hearing gunshots, discovering Finn lying on the ground with a head wound, while their son was bleeding on the ground after shooting himself in the face.

Finn's former instructor Kathy Kairns-Scholz told NY Post that her former student had a special talent for dance, saying: “She was dedicated and worked 100% all the time.

"She knew everything. She picked choreography up very quickly and had a great mind that she could retain everything. A beautiful performer.”

Lynch is being held without bail while the trial continues.

You can donate to Emily Finn's GoFundMe here.