San Francisco police Sergeant Michael Horan was honored at City Hall Wednesday as part of an Officer of the Month ceremony, but he bears the distinction of helping crack a case that made international headlines last month.

Sergeant Horan, a third-generation officer in the San Francisco Police Department, was working on the recently opened missing persons case of Luigi Mangione when, on December 5, the New York Police Department released those now infamous surveillance images of a smiling Luigi Mangione, the suspect in the December 4 assassination of United Healthcare CEO Brian Thompson.

As he tells the Chronicle in an interview, he picked up on something looking at photos submitted by Mangione's family — which he just so happened to be looking at on December 5, by coincidence, when news of those surveillance images came on CNN.

Sergeant Joe Siragusa was the officer first assigned to the missing persons case, which Kathleen Mangione, Luigi's mother, had filed on November 18. As we learned last month after his arrest, the family had not heard from him since July, and the mother had reason to believe he could still be in San Francisco, where he had claimed to be working for the car-sales startup TrueCar. He had given his mother some fictional information, like an office phone number that was non-working, and the SFPD concluded that, perhaps, he didn't want to be found by a mother his friends said could be overbearing.

Shortly after his arrest, we learned that someone in the SFPD's Special Victims Unit had ID'd the shooter as possibly the same person, Mangione, reported missing here in SF, and passed that information on to the FBI, who forwarded it on to the NYPD, and now we know that someone was Sergeant Horan.

Horan was handed the case by his partner Siragusa after the Mangione familiy repeatedly called the SFPD to follow up, asking him to take a second look.

"There were a couple of these photos [on Mangione's Instagram] where he’s smiling at just the right angle, and it just kind of dawned on me," Horan tells the Chronicle. "Like, oh my God. That smile looks exactly like the guy in the surveillance photos."

The tip hadn't even been fully vetted by the NYPD when Mangione was separately spotted at a McDonald's in Pennsylvania, and subsequently arrested by police there.

At Wednesday's City Hall ceremony, Sergeant Horan was honored for the "incredible work he's done to make San Francisco safer for all of us," and the "exceptional abilities" that led to his making the identification of Mangione.

"What started as a routine missing persons case evolved into something much greater... a critical lead in a high-profile murder case," said Acting Commander Alexa O'Brien of the SFPD, at yesterday's ceremony. "His ability to connect seemingly unrelated dots is what sets him apart as a true leader in law enforcement."

See the full ceremony below, which includes the honoring of October's Officers of the Month, Casey Chow and Anthony Quimbo; and November's Officer of the Month, Sergeant Inspector Antonio Flores.

Related: Healthcare CEO Shooting Suspect Was Reported Missing In San Francisco This Fall