FIRST ON FOX: A tip that was provided by the public to the San Francisco Police Department helped FBI investigators identify the UnitedHealth CEO suspected killer, Luigi Mangione.

According to an FBI NYC source, a tip was received from the San Francisco Police Department on the possible identity of Mangione following the Dec. 4 shooting in Midtown Manhattan.

The particular tip assisted FBI NYC during their investigation and eventual arrest of the 26-year-old Ivy League graduate.

“FBI New York conducted routine investigative activity and referred this and other leads to the New York City Police Department as part of our assistance to them in their investigation,” sources said. “Extensive sharing of the photos by law enforcement led to the identification by a citizen and subsequent arrest by the Altoona Police Department.”

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The additional information sheds more light on investigators’ path to identifying Mangione.

Fox News previously confirmed that a San Francisco police investigator recognized Mangione from a wanted bulletin poster made by police and communicated that to the FBI while the suspect was still at large.

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The poster, obtained by Fox News, shows a picture of a smiling Mangione and states that the missing person report for him was filed on Nov. 18, about two weeks before he allegedly shot and killed Thompson execution-style outside the New York Hilton Midtown hotel. The poster goes out to agencies for a missing person.

The police source tells Fox News that Mangione’s mother phoned in to file the missing person report stating she last spoke with her son around July 1 and that he worked at TrueCar, an online retailing service for car buyers and auto dealerships.

The San Francisco location given for a work address was 124 Montgomery, which is permanently closed and there is no phone number.  

Mangione’s mother said she didn’t know any other place her son would frequent in San Francisco, per the source. 

Authorities arrested Mangione in a McDonald’s in Altoona, Pennsylvania, on Monday, at which point he allegedly presented local police with a fake ID and appeared to start shaking when they asked if he had been in New York recently.

In New York, Mangione faces one count of murder, two counts of second-degree criminal weapons possession, one count of second-degree possession of a forged document and one count of third-degree criminal weapons possession.

In Pennsylvania, he faces one count of forgery, one count of carrying a firearm without a license, one count of tampering with records or identification, one count of possession of instruments of a crime and one count of presenting false ID to law enforcement, according to court documents.

Fox News Digital has reached out to the FBI for comment.

Fox News’ Matt Finn, Christina Coulter, Louis Casiano and Michael Lundin contributed to this report.