A Long Island music teacher has been accused of carrying out a years-long plan that ended in the killing of his 25-year-old sister-in-law, in a case that prosecutors describe as one of the most disturbing murder investigations in recent memory.

According to the Nassau County District Attorney’s Office, Joseph Horner, 27, allegedly spent nearly eight years planning the attack on Victoria Castle, his wife’s younger sister. Prosecutors say Horner admitted that he had been thinking about killing Castle since 2017, carefully preparing for the crime long before it occurred.

Authorities allege that Horner repeatedly entered Castle’s downstairs apartment without her knowledge over several years. During those alleged break-ins, investigators say he searched through her belongings, viewed private photographs, and became increasingly fixated on her. Prosecutors further claim he even practiced the chokehold he intended to use during the attack.

The alleged assault took place on June 28, 2025, inside Castle’s apartment in Nassau County, New York. According to investigators, Horner entered the residence and fatally attacked the young woman. Prosecutors allege that the attack also involved sexual violence. Family members reportedly became concerned after they were unable to reach Castle, leading to the discovery of the crime.

During a court appearance, prosecutors unsealed a murder indictment detailing the alleged years of planning behind the attack. Assistant District Attorney Sarah Sacks told the court that Horner “followed through on his plan that he admits he had been planning since 2017.”

Horner has been charged with first-degree murder, second-degree murder, burglary, and other related offenses. Prosecutors argue that the alleged premeditation, repeated unlawful entries, and statements made by the defendant demonstrate extensive planning rather than a spontaneous act.

The case has deeply shaken the Long Island community, where Horner worked as a music teacher. The allegations have prompted renewed discussions about stalking behavior, obsessive conduct, and the warning signs that can precede acts of extreme violence. Authorities have emphasized that the charges are allegations, and Horner is presumed innocent unless proven guilty in court.

If convicted on the most serious charges, Horner could face life imprisonment without the possibility of parole under New York law.

Source: New York Post; Nassau County District Attorney’s Office; ABC News; CBS New York.