NYPD divers were searching a body of water in New York’s Central Park on Saturday for the firearm used in the fatal shooting of a health care executive in Manhattan, a law enforcement official told CNN, as the search for the gunman nears the end of its fourth day.

The divers were seen in water known as the Lake, near the park’s iconic boathouse and Bethesda Fountain, the official said.

At least two key pieces of evidence in the Wednesday killing of UnitedHealthcare CEO Brian Thompson continued to evade investigators Saturday, including an electric bike the suspect rode and the gun used in the killing.

On Friday evening, authorities found a backpack in the park which will be tested at a forensic lab in Queens, a law enforcement official told CNN.

No gun was found in the backpack believed to have belonged to the gunman, law enforcement officials said Saturday. Authorities have yet to officially confirm the bag’s origins, an official told CNN Friday.

The Peak Design backpack contained a Tommy Hilfiger jacket, according to the officials. It was not immediately clear if other items were in the backpack, officials said.

Federal investigators are assisting the NYPD as they piece together clues, including the backpack, photos, video, ammunition, DNA and more, to help zero in on a suspect they believe is no longer in the city.

The FBI says it is offering a reward of up to $50,000 for information leading to an arrest and conviction of the gunman.

“We are seeking the public’s help in identifying the unknown suspect responsible for the homicide,” the FBI said Friday in an agency news release. The NYPD is also offering a reward of up to $10,000, the department said in a social media post Thursday.

The shooting suspect, believed to have left New York City on an interstate bus, has not been identified or located following the shooting police believe was “definitely planned,” NYPD officials told CNN on Friday.

The suspect traveled to New York City last month on a Greyhound bus that began its route in Atlanta, officials have said.

On Saturday, detectives from the NYPD arrived in Atlanta to help further the investigation, two law enforcement officials briefed on the matter told CNN. The Atlanta Police Department has said it is assisting New York police.

New York City Mayor Eric Adams on Saturday cited the backpack’s discovery as “obviously a positive development.”

“Let him continue to believe he can hide behind a mask,” Adams said of the suspect. “We’re going to reveal who he is, and we’re going to bring him to justice.”

Investigators are focused on identifying the unmasked man pictured in surveillance images released Thursday which authorities say offers a significant clue in the search. Investigators appear to be homing in on the possible identity of the suspect, two people briefed on the investigation said, but are still working to verify the information and are not yet certain they know his identity.

New York City Police Commissioner Jessica Tisch told CNN on Friday authorities have gathered “a huge amount of evidence,” including fingerprints, DNA evidence and a “massive camera canvass” of the suspect’s movements throughout the city.

Investigators are examining whether the shooter used a specialized weapon in the killing, Joseph Kenny, the chief detective of the NYPD, told reporters Friday.

“This isn’t ‘Blue Bloods.’ We’re not going to solve this in 60 minutes. We’re painstakingly going through every bit of evidence that we can come across,” Kenny said.

Law enforcement officials have briefed border guards on both sides of the US border with Canada and Mexico to be on the lookout for the suspect, a law enforcement source briefed on the investigation told CNN.

Upon his arrival in New York on November 24, the suspect took a cab from the Port Authority bus terminal to the vicinity of the Hilton Hotel and stayed there for about half an hour, he said.

Investigators say the suspect then used a fake New Jersey driver’s license to check into an Upper West Side hostel which has since notified its guests it is cooperating with the police investigation.

Photos from the hostel along with a burner phone and water bottle believed to have been dropped by the suspect upon fleeing the Wednesday shooting scene outside the Hilton Midtown have offered investigators additional clues in the case.

Kenny told reporters the words “delay,” “deny” and “depose” were scrawled on the ammunition, one word on each of three bullets. The bullets are still being tested, he said.

Police are exploring whether the words could indicate a motive, since they are similar to a phrase commonly used to describe insurance company tactics: “Delay, deny, defend.” The phrase is also the title of a book critical of the industry.

“It could possibly be a disgruntled employee or a disgruntled client,” Kenny said Friday when asked about the words on the bullets.

UnitedHealth Group, the parent company of Minnesota-based UnitedHealthcare, has mapped out security plans for employees in the wake of the attack.

“We’re ensuring the safety, security and wellbeing of our employees,” said Andrew Witty, CEO of UnitedHealth Group in an email sent to employees on Thursday and obtained by CNN.

Police appear to be homing in on suspect’s identity

NYPD investigators appear to be getting closer to a possible identification of the suspected shooter, two people briefed on the investigation said late Friday.

Investigators are working to track the suspect’s potential whereabouts with the help of thousands of investigative leads, including traces of DNA which could help find the killer, the people briefed on the probe said.

Investigators are still working to verify the information as of Saturday.

Some techniques have led to dead ends, with detectives unable to use facial recognition on the unmasked hostel photos to identify the shooter – possibly because of the angle of images or restrictions of the database, Kenny told reporters Friday. A fingerprint previously obtained by police was unusable for an identification, he said, adding it could be enhanced later in the investigation. Bus manifests likewise may not be helpful since identification is not required to board, according to Kenny.

Police have been able to use surveillance video to track the shooter’s movements after he arrived in New York City on November 24 and for a short time directly after the December 4 shooting. Investigators are processing video from the Port Authority in hopes of finding out what bus he got on if he did board one.

They have also contacted UnitedHealthcare’s security team to obtain its own person-of-interest list, looking for people who have been publicly vocal with complaints against the company, according to the source.

There’s no indication, however, that the shooter and CEO had any prior interactions or that the shooting was related to Thompson’s personal life, Kenny said in the Friday news briefing.

Police are asking about anyone who has used any vitriolic language and made threats, either on social media or through attendance at public demonstrations.

“As we’re going through our investigation, we’re coming across a lot of online comments, a lot of online threats, things of that nature. His wife indicated that he had received some prior threats,” Kenny said.

The law enforcement source said most people on the list have been already been dismissed by using social media searches. The rest are being looked into for any possible ties.

The mandate is to continue to build a video timeline of every day the suspect was in the city, the source said.

Identifying the suspect is critical and multiple law enforcement agencies are offering to help in the search once an identification is made, the source added.

How the suspect carried out the deadly shooting may suggest he practiced the attack ahead of time, former FBI Special Agent Ken Gray told CNN on Saturday.

“The fact that he was able to get to that location, be there in time, shoot Thompson and then get to his bike and get out of there and be on a bus out of the area, out of New York City as quickly as he did, that shows that he practiced this, that he knew what he was doing as far as his exit route from the area,” Gray said.

The suspected shooter still made errors that could help authorities track him down, according to Gray.

“He made the mistake of going into the Starbucks and leaving his forensic evidence behind there with the DNA, with the fingerprints,” Gray said.

“Is this guy a professional assassin? I don’t think so. But is this a person who really thought this through? I think that’s definitely the case,” he said.

Investigators carefully examining backpack believed tied to shooting

Investigators are treating the backpack found in Central Park like a dead body, being very careful not to damage any potential evidence, a law enforcement official told CNN.

The backpack was removed by an excavator along with any physical material found surrounding it, the official said. No officers picked it up, because there may be some DNA evidence tied to the backpack or even the dirt on the ground surrounding it.

“Their instructions were, ‘If you find it, freeze the location, do not touch it. Wait for the detectives, wait for the crime scene unit and let them package this up,’” CNN Chief Law Enforcement and Intelligence Analyst John Miller told Anderson Cooper on Friday.

“This is one of these things where even touching it or going through it with rubber gloves, you can smudge prints, move DNA, contaminate it with your own material,” Miller said.

“When they go to trial, they want to be able to testify this was all done with perfect procedure in terms of trying to preserve that evidence and then extract that evidence,” he added.

Forensic investigators at a police lab in Queens will go through the contents of the bag slowly and deliberately, examining not only the contents but the bag itself, which may contain sweat or other evidence connected to the shooter, the official said.

Authorities investigating whether shooter used specialized weapon

Investigators are also looking into the possibility the gunman who killed Thompson may have used what is known as a veterinary gun, a larger firearm used on farms and ranches to put down animals without causing a loud noise, Kenny, the chief detective of the NYPD, said Friday.

The veterinary gun is based on the design of a British-made Welrod pistol, dating back to World War II, was an assassin’s weapon, Miller explained.

“It’s practically silent. It’s a significant round. It’s a 9 millimeter round. And it’s meant for close-up killing. It’s meant for vets to kill an animal by shooting it in the head or somewhere, you know, that’s going to be fatal. But it would work the exact same way on humans, very effectively and extraordinarily quietly,” Miller said.