NEW YORK (AP) -- The gunman who killed the CEO of the largest U.S. health insurer likely left New York City on a bus soon after the brazen ambush that has shaken corporate America, police officials said. But he left something behind: a backpack that was discovered in Central Park.

Nearly four days after the shooting of UnitedHealthcare CEO Brian Thompson, police still did not know the gunman's name or whereabouts or have a motive for the killing. Investigators were looking at whether the shooter may have been a disgruntled employee or client of the insurer, Chief of Detectives Joseph Kenny told reporters.

The FBI announced Friday night it was offering a $50,000 reward for information leading to an arrest and conviction.

Video of the gunman fleeing Wednesday's shooting showed him riding a bicycle into Central Park and later taking a taxi to a bus terminal that offers commuter service to New Jersey and Greyhound routes to Philadelphia, Boston and Washington, D.C, according to Kenny.

Police have video of the man entering the bus station but no video of him exiting, leading them to believe he left the city, Kenny said.

Investigators on Friday found a backpack in the park that had been worn by the gunman during the shooting, police said, following a massive sweep to find it in a vast area with lakes and ponds, meadows, playgrounds and woods.

Police didn't immediately reveal what, if anything, it contained but said it would be analyzed for clues.

The gunman made sure to conceal his identity with a mask during almost all of his time in the city, including during the attack and while he ate, yet left a trail of evidence in view of the nation's biggest city and its network of security cameras.

The gunman arrived in New York City on Nov. 24 and shot Thompson 10 days later outside his company's annual investor conference at a hotel just blocks from Radio City Music Hall and Rockefeller Center.

The gunman got off a bus that originated in Atlanta and made several stops along the way, Kenny said. Police have not determined where he got on the bus. Investigators have a list of passengers, but none of them would have had to provide an ID when they climbed aboard, Kenny said.

Investigators believe the suspect used a fake identification card and paid cash, Kenny said, when he checked in at the hostel, which has a café along with shared and private rooms and is blocks from Columbia University.

Investigators have tested a discarded water bottle and protein bar wrapper in a hunt for his DNA. They also were trying to obtain additional information from a cellphone found along the gunman's escape route.

Photos of the suspected shooter that were taken in the lobby of a hostel on Manhattan's Upper West Side appear to from the only time he removed his mask, Kenny said. The images, showing a man smiling in the lobby of the HI New York City hostel. They are among a collection of photos and video circulated since the shooting -- including footage of the attack, as well as images of the suspected gunman at a Starbucks beforehand.

"From every indication we have from witnesses, from the Starbucks, from the hostel, he kept his mask on at all times except for the one instance where we have him photographed with the mask off," Kenny said.

His roommates at the hostel also said he didn't speak to them. Nothing of investigative value was found in a search of the suspected shooter's hotel room.

Asked how close he felt police were to making an arrest, Kenny said, "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. Eventually, when an apprehension is made, we will have to present all of these facts to a judge and jury, so we're taking our time, doing it right and making sure we're going to get justice for this victim and closure for his family."

Security video of the shooting shows the killer approaching Thompson from behind, firing several shots with a gun equipped with a silencer, barely pausing to clear a jam while the executive fell to the sidewalk.

Police were looking into the possibility that the weapon was a veterinary pistol, which is a weapon commonly used on farms and ranches if an animal has to be euthanized quietly, Kenny said -- though he stressed that hadn't been confirmed.

The words "delay," "deny" and "depose" were scrawled on the ammunition, one word on each of three bullets, Kenny said. A law enforcement official previously told The Associated Press the words were "deny," "defend" and "depose." The messages mirror the phrase "delay, deny, defend," which is commonly used by lawyers and critics about insurers that delay payments, deny claims and defend their actions.

Thompson, a father of two sons who lived in a Minneapolis suburb, had been with Minnesota-based UnitedHealthcare since 2004 and served as CEO for more than three years.

The insurer's parent company, UnitedHealth Group Inc., was holding its annual meeting in New York for investors. The company abruptly ended the conference after Thompson's death.

UnitedHealth Group said it was focused on supporting Thompson's family, ensuring the safety of employees and assisting investigators. "While our hearts are broken, we have been touched by the huge outpouring," the company said.

UnitedHealthcare provides coverage for more than 49 million Americans. It manages health insurance coverage for employers and state and federally funded Medicaid programs.

In October, UnitedHealthcare was named along with Humana and CVS in a Senate report detailing how its denial rate for prior authorizations for some Medicare Advantage patients has surged in recent years.

The shooting has rocked the health insurance industry in particular, causing companies to reevaluate security plans and delete photos of executives from their websites. A different Minnesota-based health care company said Friday it was temporarily closing its offices out of an abundance of caution, telling employees to work from home.

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Balsamo reported from Washington. Jake Offenhartz, Cedar Attanasio and Karen Matthews in New York, John Seewer in Toledo, Ohio, and Jeff Martin in Atlanta contributed to this story.