As the manhunt for the masked man who fatally shot UnitedHealthcare CEO Brian Thompson on a New York City sidewalk entered its fourth day on Saturday, the FBI offered a $50,000 reward for tips on the identity of the killer.

No new breakthroughs in the search have been announced since Friday evening when law enforcement officers discovered a backpack in Central Park believed to have belonged to the suspected killer.

According to the New York Post, Mayor Eric Adams told reporters at a police holiday party on Saturday that the "net is tightening." Authorities believe the suspect in Wednesday's shooting left the city on a bus.

Thompson, who was gunned down outside the New York Hilton Midtown, was in New York to attend an investors' conference for UHC's parent company, UnitedHealth Group, a health insurer and the nation's fourth-largest company, with headquarters in Minnetonka.

Investigators have relied on the web of video cameras spread out throughout New York City to locate Thompson's suspected killer. Officials say the gunman arrived in New York on Nov. 24 on a bus that originated in Atlanta. Video footage captured the man briefly removing his mask while checking into an Upper West Side hostel and purchasing a water bottle and snack at a Starbucks on the morning of the shooting.

After shooting Thompson, the suspect sailed off into Central Park on a bike. Video later tracked him entering a bus station on the northwestern edge of Manhattan.

The cold-blooded shooting of Thompson, a 50-year-old father of two who lived in Maple Grove, drew reactions of shock and outpourings of grief from across the nation, including public officials, colleagues and Thompson's own community in the west metro suburbs.

But the killing of the insurance executive, who once led a business unit that has been scrutinized for denying health care claims, has sparked a public debate about the power that insurers wield in the health care system.

In a video address to UnitedHealth Group employees on Friday, CEO Andrew Witty called the killing "immeasurably sad" and "profoundly shocking," and encouraged workers to care for one another.

"I'd like to ask you all to look out for yourselves," Witty said. "This is a moment where, in among everything else, we're reminded for the fragility of families, of individuals and the importance that that really represents."