Joel Seidemann: Manhattan DA Cases and the Mangione Prosecution

Joel Seidemann is the veteran senior trial counsel in the Manhattan District Attorney’s office who is leading the state murder prosecution of Luigi Mangione, the man charged with the December 2024 killing of UnitedHealthcare CEO Brian Thompson.¹ He has been with the office since May 1982 and has held the senior trial counsel role since 1989, trying murder cases and other serious felonies for most of that time.²

Four Decades in the Manhattan DA’s Office

Seidemann was admitted to the New York bar in 1980 and joined the Manhattan DA’s office two years later.³ A George Washington University Law School graduate, he rose to senior trial counsel by 1989 and has spent the bulk of his career there.² Former colleague Daniel Bibb has said Seidemann has “probably tried more cases than anybody in the DA’s office.”¹

Colleagues and opposing counsel describe him as a “firecracker” with a knack for turning dry police and medical reports into narratives juries follow. Former colleague Elizabeth Loewy has said, “He’s not afraid to call it what it is.” Defense attorneys who have faced him call him a “relentless adversary” but also “tough but fair.”¹ He also has a specific reputation for dismantling psychiatric defenses, and that reputation reportedly factored into his assignment to the Mangione case.

The Cases That Built His Reputation

Brooke Astor’s Son

In 2009, Seidemann led the prosecution of Anthony Marshall, philanthropist Brooke Astor’s son, for looting his mother’s nearly $200 million estate while she suffered from Alzheimer’s disease. The six-month trial featured nearly 70 witnesses, including Henry Kissinger and Barbara Walters. After 12 days of deliberation, a Manhattan jury convicted Marshall on 14 of 16 counts, including first-degree grand larceny and scheming to defraud. His co-defendant, attorney Francis Morrissey, was convicted on all five counts against him, including forgery.⁴ Seidemann called the defendants “two morally depraved individuals.”

Etan Patz

Seidemann helped secure one of New York’s most long-awaited convictions in the case of Etan Patz, the six-year-old who vanished in lower Manhattan in 1979 and became a national symbol of missing children. A first trial of Pedro Hernandez ended in a mistrial. At the 2017 retrial, Seidemann and co-prosecutor Joan Illuzzi delivered the closings, and the jury convicted Hernandez of kidnapping and murder.⁵

Barbara Kogan

In another cold case, Seidemann prosecuted Barbara Kogan for orchestrating the 1990 contract killing of her husband, real estate figure George Kogan, who was shot outside his companion’s Upper East Side apartment. Prosecutors alleged she conspired with her divorce attorney, Manuel Martinez, to hire a hitman, motivated by a bitter divorce and a $4.3 million insurance payout. In April 2010, Kogan pleaded guilty to manslaughter, conspiracy, and grand larceny and was sentenced to 12 to 36 years.⁶⁷

The David Lemus Loss

The most prominent blemish on Seidemann’s record is the case of David Lemus, stemming from a 1990 shooting at Manhattan’s Palladium nightclub that killed a 23-year-old bouncer. Lemus and co-defendant Olmedo Hidalgo were convicted of second-degree murder in 1992. Questions mounted after Joseph Pillot confessed in 1995 to committing the crime with Thomas Morales, and by 2004 the DA’s own Cold Case Unit concluded that “substantial new evidence indicates that Lemus and Hidalgo were NOT involved.” Hidalgo’s conviction was dismissed, and Lemus was released in 2005 after a judge granted him a new trial.⁸ At that December 2007 retrial, Lemus was acquitted. The verdict was characterized in press coverage as a “sharp rebuff” to the Manhattan DA’s office for continuing to pursue the case even after the exculpatory evidence emerged.⁹ Lemus had spent nearly 14 years in prison.

Prosecuting Luigi Mangione

Seidemann is leading the state prosecution of Mangione, 28, for the December 4, 2024, fatal shooting of Brian Thompson in midtown Manhattan. He is trying the case alongside assistant district attorneys Kristin Bailey and Zachary Kaplan.¹⁰ Mangione has pleaded not guilty to both state and federal charges.¹¹

The Indictment

A Manhattan grand jury indicted Mangione on 11 counts. The most serious were first-degree murder alleging the killing was in furtherance of an act of terrorism, and second-degree murder as a crime of terrorism. The indictment also included a standalone count of second-degree murder, multiple weapons counts covering a 3D-printed firearm, a silencer, prohibited magazines, and a ghost gun, and criminal possession of a forged instrument for a fake New Jersey driver’s license.¹² Judge Gregory Carro later dismissed the terrorism enhancement from the murder charges, a pretrial win for the defense.¹³

The Suppression Fight

The central pretrial battle involved evidence seized during Mangione’s December 9, 2024, arrest at a McDonald’s in Altoona, Pennsylvania. After a nine-day suppression hearing in late 2025, Judge Carro issued a split ruling in May 2026. He found the initial warrantless search of Mangione’s backpack at the restaurant improper and suppressed items pulled directly from it, including a cellphone, passport, wallet, and a loaded magazine. But he ruled that a subsequent inventory search at the Altoona police station followed proper protocols, allowing the prosecution to use its most critical evidence: the 3D-printed 9mm pistol identified as the murder weapon and a notebook containing writings about wanting to “wack” a health insurance executive.¹⁴¹⁵

The hearings were contentious. Seidemann and lead defense attorney Karen Friedman Agnifilo clashed over terminology throughout: the defense called the notebook a “journal” while Seidemann called it a “manifesto,” and the defense objected to his use of the word “execution” to describe the killing. Judge Carro at one point admonished him, sustaining an objection and warning, “He certainly won’t do that at trial.” Seidemann replied, “I’ll bear that in mind, your honor.”¹⁶

Forcing the Psychiatric Defense Out

Mangione’s team initially filed notice of an “extreme emotional disturbance” defense, a strategy that, if successful, could reduce a murder conviction to first-degree manslaughter and cap the maximum sentence at 25 years instead of life. The defense intended to argue that Mangione’s hostility toward UnitedHealthcare and the for-profit healthcare system drove him into a psychiatric state that deprived him of rational thought.¹⁷

Seidemann pushed hard against it. He accused the defense of “stonewalling” by withholding psychiatric records and expert information, telling the court, “We have gotten nothing.” He successfully pressured Judge Carro to order early disclosure of the defense’s psychiatric expert and the specific condition it planned to claim, with the judge warning, “I’m not going to let you surprise the People on the eve of trial.”¹⁸¹⁹

On June 18, 2026, the defense reversed course and filed a letter withdrawing its psychiatric defense notice. Legal analysts noted the move avoided the immediate deadline to produce psychiatric records but does not legally foreclose the defense from attempting such an argument later, though doing so would be far more difficult.¹⁸

Scheduling and the Federal Case

Mangione faces parallel prosecutions. The state case is the one charging him with murder. The federal case, brought by the Southern District of New York, charges him with stalking and murder under federal jurisdiction because he allegedly crossed state lines to commit the crime, and federal prosecutors have filed notice of intent to seek the death penalty.²⁰

Scheduling produced what Agnifilo called a “tug of war between two different prosecution offices.” The state trial was originally set for June 2026 but was postponed to September 8, 2026, after the defense argued it could not manage two complex trials at once.²¹ Seidemann pushed back against delays, filing a letter requesting a July 1 trial date and arguing that “the State has an overriding interest in trying this defendant for the cold-blooded execution of Brian Thompson.”²² He also told the court that Brian Thompson’s mother had asked federal prosecutors to let the state murder trial go first.²³ Federal and state authorities ultimately agreed the state case would proceed first, with the federal trial scheduled for October 2026 and jury selection beginning October 5.

Courtroom Security

Seidemann has raised concerns about the courtroom atmosphere, citing fears about “people who may be making threats” among Mangione’s vocal base of public supporters. He has indicated prosecutors may ask Judge Carro to seat an anonymous jury and to bar courtroom attendees from wearing pins or T-shirts, or carrying signs, showing support for the defendant during trial.²⁴

1
Business Insider. Who Is Luigi Mangione’s Prosecutor, Veteran Lawman Joel Seidemann
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HarperCollins. Joel Seidemann
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Justia. Joel Jonathan Seidemann
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CNN. Anthony Marshall Found Guilty in Astor Case
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The New York Times. Etan Patz Case – Pedro Hernandez
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The New York Times. Woman Sentenced for Having Husband Killed
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New York Daily News. Barbara Kogan Admits Role in 1990 Murder
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NBC News. Palladium Nightclub Shooting Case
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The New York Times. Palladium Nightclub Case Acquittal
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Manhattan District Attorney’s Office. D.A. Bragg Announces Murder Indictment of Luigi Mangione
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WSLS. Luigi Mangione’s Hearing Delayed a Day
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Manhattan District Attorney’s Office. Mangione Indictment
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ABC News. Luigi Mangione Returning to Court
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PBS NewsHour. Judge Allows Gun and Notebook as Evidence in Luigi Mangione Murder Trial
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CNN. Luigi Mangione Evidence Trial
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Business Insider. Luigi Mangione in Court – War of Words Evidence Hearing
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Courthouse News. In Luigi Mangione’s Psychiatric Defense, the American Healthcare System Is on Trial
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NPR. Mangione Psychiatric Defense Update
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Courthouse News. Luigi Mangione Will Make Extreme Emotional Disturbance Defense at New York Murder Trial
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New York Courts. People v. Luigi Mangione Omnibus Decision
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The Guardian. Luigi Mangione New York Court Postpone Federal Trial
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NBC News. Prosecution Requests July 1 State Trial for Mangione
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The New York Times. Luigi Mangione UnitedHealthcare Manhattan Trial
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The Guardian. Luigi Mangione Trial Defense Strategy