Before he was accused of fatally stabbing a Rome police officer late last week, a former San Francisco high school student punched a fellow student at a 2016 late-night party in Stern Grove and left the victim with a severe brain injury, sources familiar with the matter told The Chronicle.

Finnegan Lee Elder is now accused of stabbing Vice Brigadier Mario Cerciello Rega 11 times on Friday, leaving the Carabinieri paramilitary police officer bleeding profusely in what Rome authorities said resulted from an investigation into a bungled cocaine deal.

Elder, 19, and his 18-year-old friend and travel companion, Gabriel Christian Natale-Hjorth, also of San Francisco, are now jailed in Rome in the slaying. Elder allegedly admitted to the stabbing, according to Italian authorities, but told a judge on Saturday that he stabbed the plainclothes officer out of fear he was being strangled, t he Associated Press reported .

The tragedy has reverberated in Italy and beyond with support pouring in for the 35-year-old fallen officer, who returned from his honeymoon days before he died, and whose funeral was held Monday in the same church where he was married six weeks earlier.

The young men held in the slaying graduated from Tamalpais High School in Mill Valley in 2018. Elder, though, had attended Sacred Heart Cathedral Preparatory in San Francisco and was a member of the football team when he punched a fellow student and football player during a party on Oct. 29, 2016, sources familiar with the investigation said.

Elder, who was 16 years old at the time of the incident, struck another 16-year-old student in the wooded park near Sloat Boulevard and 19th Avenue that’s known as a nighttime gathering spot for teens, sources said.

The victim struck his head and was hospitalized with “life-threatening injuries,” police told The Chronicle at the time. The victim experienced a long recovery and has since graduated from high school and is attending college. He could not be reached for comment.

Elder turned himself in after the 2016 incident and was among around a dozen players suspended for the final game of the season at Kezar Stadium, according to sources and an interview that year with the school’s director of athletics.

Officials at Sacred Heart did not reply to messages from The Chronicle. The school did not comment on the incident at the time and it’s not clear Elder faced discipline before he began attending Tamalpais High.

Elder was arrested on suspicion of battery involving serious bodily injury and his case was adjudicated in juvenile court, sources said. Sources said the court found that he committed a felony offense in the incident but it’s not clear what punishment he faced. The sources spoke on the condition of anonymity due to the sensitivity of the case.

The San Francisco district attorney’s office said it was unable to comment on juvenile cases. San Francisco police also declined to comment.

Sean Elder, Finnegan Elder’s uncle, who is acting as a family spokesman, told The Chronicle that the incident was part of “a mutual pre-agreed upon fight, which many football team members knew about and egged on.”

“I've known Finn for his entire life and have never seen him be violent, or even lose his temper,” Sean Elder said. He added that the school did not discipline his nephew. He said the teen transferred to Tamalpais High after his father moved to Mill Valley.

The family said in a statement Monday that it plans to travel to Rome soon and that a U.S. government official had visited Finnegan Elder. “We continue to gather facts about his case through his legal representatives. Meanwhile, we are grateful that he has been provided medical care,” the family said.

Over the weekend, a judge in Rome ordered the Americans to be kept behind bars as the investigation into the death continues. Finnegan Elder and Natale-Hjorth were apprehended hours after the stabbing and police said they found a military-style knife used in the killing hidden in their hotel room’s ceiling.

Judge Chiara Gallo said Elder didn’t have any marks on his neck after telling authorities he stabbed Cerciello Rega out of fear he was being strangled, according to a transcript of the judge’s ruling.

The judge wrote that Elder told investigators he didn’t realize Cerciello Rega and his partner, Andrea Varriale, were police officers and believed they were connected to a man whose bag and cell phone they took hours earlier while trying to arrange a drug deal.

The man whose bag they allegedly stole, identified as Sergio Brugiatelli, told police the two men approached him in Rome’s Trastevere district asking to purchase cocaine, the judge said. Brugiatelli then accompanied Natale-Hjorth to a dealer in the neighborhood while Elder waited with Brugiatelli’s bag and cell phone on a bench.

Approaching police, though, caused the group to scatter after Natale-Hjorth gave the dealer money for drugs. When Brugiatelli returned to the bench, Elder had run off with his bag and cell phone, the judge wrote.

When Brugiatelli called his cell phone number, the Americans allegedly demanded he bring 80 or 100 euros plus a gram of cocaine to a street near their hotel in exchange for his bag. Police learned about the alleged shakedown and Cerciello Rega and Varriale headed to the rendezvous point, the judge said.

The judge said Varriale told investigators they showed their badges and identified themselves as police before they were attacked by the Americans.

Police and prosecutors said they were investigating after Italian newspapers published a photo Sunday of Natale-Hjorth with a scarf covering his eyes and his hands handcuffed behind his back inside the police station.

Rome officials called the blindfolding “illegal” but said it lasted only a few minutes before Natale-Hjorth was interrogated. Rome’s prosecutor general said that a lawyer was present during the Americans’ interrogation.

The Associated Press contributed to this report from Rome.