The manhunt for the suspect involved in gunning down UnitedHealthcare CEO Brian Thompson outside a Midtown Manhattan hotel on Wednesday has now entered its fourth day as authorities learn more about his movements before the attack.

Police have learned that the suspect took a taxi to the Port Authority bus facility at 178th Street and boarded a bus out of New York City following the shooting, according to police.

The bus the suspect is believed to have boarded out of the city made six or seven stops, and investigators have followed leads in multiple states, the sources said.

The killer entered New York City by bus on Nov. 24, when a surveillance camera at Port Authority Bus Terminal caught his arrival at 9 p.m., law enforcement sources told ABC News.

The inbound bus originated in Atlanta, but it was not immediately clear where the suspect boarded. Sources told ABC News he was spotted on board in Washington, D.C.

The 10-day period he was in New York City before the shooting is the focus of investigative efforts.

The FBI is now assisting in a nationwide manhunt for the suspect, according to law enforcement sources, and the unidentified man remains at large in the wake of Wednesday's attack, which was described by police as "brazen, targeted" and "premeditated."

Police still do not have a name of the suspect, the sources said.

Meanwhile, citing the recent killing of Thompson, Centene, a major health insurer, said it would be moving its planned in-person investor day in New York City next week to virtual, according to a statement from the company on Friday.

"All of us at Centene are deeply saddened by Brian Thompson's death and want to express our support for all of those affected. Health insurance is a big industry and a small community; many members of the CenTeam crossed paths with Brian during their careers," said Centene Chief Executive Officer, Sarah M. London. “"He was a person with a deep sense of empathy and clear passion for improving access to care. Our hearts are with his family and his colleagues during this difficult time."

Police officers were able to recover what is suspected to be the backpack carried by the suspected shooter, a source familiar with the case told ABC News on Friday.

The bag was discovered Friday evening in Central Park, where the NYPD deployed an army of officers and drones to conduct a grid search, the source said.

The suspect is seen in footage wearing a backpack prior to the shooting but does not appear to have it on in another video of him on a bike 15 minutes after the shooting.

The masked gunman shot Thompson at point-blank range at 6:44 a.m. Wednesday outside the New York Hilton Midtown, where Thompson was heading for his company's investors conference.

"The shooter then walks toward the victim and continues to shoot," NYPD Chief of Detectives Joseph Kenny said. "It appears that the gun malfunctions, as he clears the jam and begins to fire again."

Written on the shell casings were the words “deny,” “defend” and “depose,” according to police sources.

The words on the bullets echo the title of the 2010 book "Delay, Deny, Defend: Why Insurance Companies Don't Pay Claims and What You Can Do About It." Police are aware of the similarity, and are investigating whether one possible motive is anger at the insurance industry, sources said.

Investigators believe they were able to score DNA samples from several pieces of evidence discovered at or near the crime scene, law enforcement sources told ABC News. The samples are currently at the NYC Office of the Chief Medical Examiner to be run through databases for a possible match -- a process that could take several days, the sources said.

Police were also able to extract a fingerprint off a water bottle the suspect bought at a Starbucks, but the print is smudged so it's not clear how helpful it will be, sources said.