Ex-star athlete accused in Princeton brother's grisly murder returns to court as prosecutors prep indictment

The Princeton, New Jersey, man known as a star student in high school, who is now accused of killing his brother and a family cat in a "fit of madness" last month, appeared in court Monday afternoon as prosecutors prepare to take his case to a grand jury.

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.

During Monday's hearing in Mercer County Superior Court, prosecutors expressed plans to take Hertgen's case to a grand jury to seek an indictment. He has a probable cause hearing scheduled for May 9.

Earlier this month, Hertgen consented to pretrial detention, meaning the prosecution didn't need to show evidence proving why he should be held in jail before his trial.

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Appearing virtually from jail, Hertgen was polite during his brief appearance, telling the judge "good afternoon" when asked if he could hear the proceedings and saying "thank you" when the judge told him to have a good day as the hearing ended.

The judge referenced a cellphone and laptop that were seized and password protected, but no other evidence was discussed on Monday.

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

When a dispatcher asked Matthew how his brother was injured, he said, "I’m not gonna go into any more details right now," a probable cause statement obtained by Fox News Digital reads.

When police arrived to the grisley scene that night, they found Matthew in the residence along with his brother's body, which had significant injuries to the head – including a missing eye  a–s well as injuries to Joseph's chest and upper body. Police 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 told police he "went into a fit of madness… maybe like forty minutes ago."

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Homicide detectives also interviewed Matthew's other surviving brother, who told police the 31-year-old suspect had struggled with mental health issues for about five years. 

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The brother had seen Matthew earlier in the day, when he had been experiencing a depressive episode and "terrifying visions," the brother told police, according to court documents.

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Hertgen appeared in court on March 6 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.

However, during that 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 for that hearing. He is scheduled to return to court on March 24.

"It's very disturbing. You don't see many murders like this," Gelman told Fox News Digital. "You can see it's very personal.… I was a prosecutor and I dealt with a ton of murders… and as a defense attorney, I also deal with murders. And when you see these types of really gruesome, graphic, heinous crimes… I have a feeling that this individual definitely has some type of mental illness."

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Gelman, who is not involved in the case, also said he would request a mental health evaluation for the suspect.

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.

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"If I was [the] defense attorney, I wouldn't fight it. I would say, you know what? We think there's a mental issue," Gelman said.

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