She also confirmed fingerprints recovered in NYC have been matched to Mangione's in Pennsylvania.
"First, we got the gun in question back from Pennsylvania. It's now in the NYPD crime lab. We were able to match that gun to the three shell casings that we found in Midtown at the scene of the homicide. We were also able in our crime lab to match the person of interest's fingerprints with fingerprints that we found, both on the water bottle, and the kind bar near the scene of the homicide in midtown," Tisch said
It represents the first forensic tie between the murder of UnitedHealthcare CEO Brian Thompson and Mangione.
Among the writings recovered from Mangione in a spiral notebook were plans concocting how to eventually kill the UnitedHealthcare CEO, according to law enforcement officials.
"What do you do? You whack the CEO at the annual parasitic bean-counter convention," one passage allegedly said.
The notebook also makes reference to the Unabomber, but the writings also said that using explosives could "risk innocents," according to the officials.
Detectives are still examining Mangione's writings but are considering the contents of the notebook to represent a confession, sources say.
Pennsylvania investigators have also started interviewing members of Mangione's family.
Earlier Wednesday morning, the defense attorney representing Mangione said he has seen "no evidence" linking his client to the killing, adding that "a lot of guns look the same."
"I have not been made aware of any evidence that links the gun that was found on his person to the crime so these are things that we're looking to see," attorney Thomas Dickey told ABC News' "Good Morning America" on Wednesday.
"Today's another day," he added. "We're looking forward to beginning our inquiry as to what evidence may or may not be out there."
Mangione, 26, plans to challenge his extradition from Pennsylvania to New York, where he faces a charge of second-degree murder in connection with Thompson's Dec. 4 outside a Midtown Manhattan hotel.
Mangione was arrested on Monday in Altoona, Pennsylvania, and charged in Pennsylvania for allegedly possessing an untraceable "ghost" gun."
"He has constitutional rights and that's what he's doing" in challenging the interstate transfer, Dickey told reporters on Tuesday.
Police officials in New York have not said whether the gun recovered in Pennsylvania is considered a match for the one used in the killing last week, but said it looks similar and that it would undergo ballistic testing.
"A lot of guns look the same," Dickey said on Wednesday. "If you brought a gun in and said, 'Well, it looks like that,' I don't even know if that evidence would be admissible. So I would argue it wouldn't be given much weight."
He also cautioned that anyone speculating on the case should take the potential evidence "in its entirety," not taking pieces of writing or other evidence "out of context."
"People put out certain things, parts of different things," he said. "I think any lawyer involved in this situation would want to see it all."
Mangione plans to plead not guilty to the charges filed against him in Pennsylvania, Dickey said. A judge has ordered Mangione held without bail.
The Manhattan District Attorney's Office said it will seek a governor's warrant to try to force Mangione's extradition. New York Gov. Kathy Hochul said in a statement that she'll sign a request for the governor's warrant "to ensure this individual is tried and held accountable."
In his first comments since Mangione was charged with second-degree murder on Monday, Manhattan District Attorney Alvin Bragg said he was confident that the 26-year-old would quickly be extradited to New York where he will be held accountable for brazenly killing Thompson.
"We will get the defendant here and bring him to justice through our court proceeding," Bragg told ABC News' Aaron Katersky.
One week after what he called a "profoundly disturbing" killing, Bragg said that his office is continuing to investigate the incident and learn more information, which could result in additional charges beyond the second-degree murder charge.
"We intend to learn more, and we have a lot of things to do," Bragg said. "As things go forward, there may be additional charges to bring."
Dickey said his client intends to plead not guilty to the charges, claiming there is no evidence linking his client to the crime. Bragg said he was confident Mangione is the person who fired three rounds into the torso of Thompson last week, killing the healthcare executive and setting off a firestorm of vitriol online.
"We would not charge the person if we didn't think it was the person, and we're prepared to go forward and we're on the path to accountability and justice," Bragg said.
Mangione yelled to the press about "an insult to the intelligence of the American people" as he was physically dragged into the courthouse on Tuesday.
Luigi Mangioni shouts as he arrives at the Blair County Courthouse
When Mangione was arrested on Monday, he had "written admissions about the crime" with him, according to the New York arrest warrant.
Mangione's writings, obtained by ABC News, addressed to the "Feds," said, "I do apologize for any strife of traumas but it had to be done. Frankly, these parasites simply had it coming."
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He claimed that the U.S. has the most expensive health care system in the world, but ranks about No. 42 in life expectancy. He said UnitedHealthcare "has grown and grown, but as our life expectancy? No the reality is, these [indecipherable] have simply gotten too powerful, and they continue to abuse our country for immense profit."
Mangione appears to have been inspired by the Unabomber, according to an NYPD intel analysis report obtained by ABC News.
The report warned that like Ted Kaczynski -- whose 17-year bombing campaign killed three and injured 23 people -- Mangione may become a "martyr" who inspires "a wide range of extremists" to act.
A grandson of a wealthy, self-made real estate developer and philanthropist, Mangione is a cousin of a current Maryland state legislator. Mangione was valedictorian at his elite Baltimore prep school, where his 2016 graduation speech lauded his classmates' "incredible courage to explore the unknown and try new things."
He went on to earn undergraduate and graduate degrees in computer science in 2020 from the University of Pennsylvania, a spokesperson said.
"Our family is shocked and devastated by Luigi's arrest," Mangione's family said in a statement posted on social media late Monday by his cousin, Maryland lawmaker Nino Mangione. "We offer our prayers to the family of Brian Thompson and we ask people to pray for all involved."
The mother of Mangione filed in November a missing person report in San Francisco, seeking info on the 26-year-old, according to two law enforcement sources.
Repeated requests for information about the report went unanswered by the San Francisco Police Department. Both the chief and the spokesman referred questions to the NYPD.
Luigi Mangione will fight extradition to New York; judge denies bail
ABC News' Aaron Katersky, Emily Shapiro, Kevin Shalvey, Peter Charalambous and Josh Margolin along with some information from the Associated Press contributed to this report.
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How police tracked down CEO murder suspect Luigi Mangione
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