The surviving brother of Princeton murder suspect Matthew Hertgen sent a warning text message to his youngest brother about an hour before he was murdered.

Matthew Hertgen, 31, is accused of murdering his 26-year-old brother, Joseph Hertgen, in the family’s Princeton apartment, allegedly with a knife and golf club. He is also charged with animal cruelty related to the death of a cat.

Matthew’s surviving brother allegedly spent time with him hours before the alleged crime on Feb. 22, a Saturday, according to a probable cause statement obtained by Fox News Digital. 

The brother apparently told police that Matthew had “suffered from severe mental illness for the past five years.”

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“He further explained that when he spoke to Matthew earlier that day, Matthew was extremely distressed, despondent, and experiencing terrifying visions,” the document states. “Wanting to help him, [the surviving brother] picked Matthew up at [redacted] at approximately 4:30 p.m. [The surviving brother] and Matthew went to the grocery store and then to [the surviving brother’s] residence … where they remained until approximately 10:00 p.m.”

The brother then drove Matthew back to his residence, where he saw Joseph Hertgen, the victim. After a brief discussion, the surviving brother left and returned to his apartment.

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Approximately an hour before the murder, the surviving brother told Joseph via text that Matthew was struggling with “his visions” and to contact him if he needed help. Joseph responded, “Will do.”

Approximately 45 minutes before Matthew called 911 to report Joseph’s dead body, his surviving brother texted him, inviting him to go on a hike the next day.

“Hang in there,” Matthew’s older brother texted him.

Around 11:16 p.m. on Feb. 22, police responded to a 911 call from Matthew reporting a fire and his brother’s dead body at the Michelle Mews Apartments complex.

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When a dispatcher asked Matthew how his brother was injured, he said, “I’m not gonna go into any more details right now,” the probable cause statement reads.

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Police arrived and found Matthew in the residence along with his brother’s body, which had significant injuries to the head — including a missing eye — as well as injuries to Joseph’s chest and upper body. They also found a cup of blood with “blood-smeared” utensils and a plate, the probable cause document states.

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Investigators further discovered a deceased cat on an ottoman that appeared partially burned.

While interviewing Matthew, who apparently had cuts on his hands, he reportedly told police he “went into a fit of madness… maybe like forty minutes ago.”

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Hertgen appeared in court Thursday morning for a detention hearing, which is typically when evidence is presented by the defense and prosecutors to determine whether a defendant should be held behind bars or released, according to David Gelman, CEO of Gelman Law and a former prosecutor in New Jersey.

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However, during Thursday’s hearing, no evidence was presented because Hertgen consented to pretrial detention, meaning it was not necessary for the prosecution to show evidence proving why he should be held in jail before his trial.

None of Hertgen’s family members were in court on Thursday. He is scheduled to return to court on March 24.

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The public defender’s office, which is representing Hertgen, told Fox News Digital that its attorneys do not comment on pending cases.

Joseph Hertgen was pronounced dead at the scene, and his autopsy results are pending. Local and state officials are investigating the 26-year-old’s death as a homicide.

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Joseph and Matthew Hertgen were star soccer players in high school and college. They were both athletes at Toms River High School, and Matthew went on to study at Wesleyan University in Connecticut. Joseph Hertgen was recruited to play soccer at the University of Michigan.