Prior to the scheduling changes, federal prosecutors and defense lawyers had explored resolving the case without a trial.
Sources familiar with the matter told ABC News that the two sides held discussions regarding a potential guilty plea.
In a joint letter, the defense and prosecution stated they would be prepared to address questions about proposed juror questionnaires and scheduling matters at the June 29 conference.
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The defense team strongly criticized the leakage of the confidential plea negotiations.
Agnifilo released a statement accusing law enforcement of attempting to influence public perception ahead of the trials.
"This information attributed to 'anonymous sources' is part of a troubling, deliberate pattern by prosecutors and law enforcement to prejudice Luigi, manipulate public opinion, and violate his constitutional right to a fair trial and impartial jury," she said.
Agnifilo further emphasized the unique burden placed on her client due to the dual state and federal prosecutions.
"Every defendant in America is presumed innocent until proven guilty, including Luigi, who, unlike any other defendant, has to fight the same charges twice," she said.
Mangione previously expressed his own frustration regarding the dual prosecutions during a state court hearing in February.
He argued that facing two separate trials for the same underlying incident violated basic legal principles.
"It's the same trial twice. One plus one is two.
Double jeopardy by any commonsense definition," Mangione said.
The state trial will proceed first in September 2026 before Judge Gregory Carro.
Mangione faces second-degree murder charges carrying a maximum penalty of 25 years to life in prison.
Mangione is accused of stalking and fatally shooting UnitedHealthcare CEO Brian Thompson outside a Midtown Manhattan hotel on December 4, 2024.
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The 28-year-old defendant has pleaded not guilty to both state and federal charges.
