The unidentified man suspected of gunning down UnitedHealthcare CEO Brian Thompson outside a Midtown Manhattan hotel remains at large after Wednesday's attack, which was described by police as "brazen, targeted" and "premeditated."

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 suspected boarded.

Police are investigating whether the suspect left New York City by bus on Wednesday after the murder.

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

Police have collected video of the suspect all over the city, including in the subway, in cabs and in a McDonald’s, sources told ABC News. In each place, he paid with cash and he made sure to keep his mask on, which indicates to detectives he knew he was coming to New York City to commit the murder, sources said.

Police were able to find a surveillance image of the suspect without his face mask on because he was flirting with the woman who checked him into the hostel, police sources said.

As he stood at the check-in desk, the sources said the woman asked to see his smile. The shooter obliged, pulling down his mask long enough for the surveillance camera to capture his face.

Police have determined the suspect checked into the hostel using a New Jersey license that wasn’t his own, police sources told ABC News.

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.

The suspect fled on foot into an alley, where a phone believed to be linked to the suspect was later recovered, police sources said.

He then fled north on a bike and rode into Central Park, police said.

A person appearing to be the suspect was seen just before 7 a.m. on the Upper West Side, riding a bicycle away from Central Park. He has not been spotted since.

UnitedHealth Group said in a statement that its "hearts are broken" and that they have been "touched by the huge outpouring of kindness and support."

"So many patients, consumers, health care professionals, associations, government officials and other caring people have taken time out of their day to reach out. We are thankful, even as we grieve,” UnitedHealth Group, parent company of UnitedHealthcare, said. "Our priorities are, first and foremost, supporting Brian’s family; ensuring the safety of our employees; and working with law enforcement to bring the perpetrator to justice."

"We, at UnitedHealth Group, will continue to be there for those who depend upon us for their health care," the statement continued. "We ask that everyone respect the family’s privacy as they mourn the loss of their husband, father, brother and friend."