Investigators are still trying to identify the suspect who shot and killed UnitedHealthcare CEO Brian Thompson outside of the New York Hilton Midtown on Wednesday (December 4) morning.

Officials said that the shooting of the 50-year-old CEO was a "premeditated, preplanned targeted attack."

According to a report from NBC News, the suspect is believed to have traveled from Atlanta to New York City on a Greyhound bus last month. Investigators are scouring through ticket records from November 24, trying to see if they can identify the man. They are also looking into reports that he used a fake ID and paid in cash when renting a room at a hostel on the Upper West Side of Manhattan.

Authorities have not determined a motive for the brazen shooting, in which the gunman opened fire on Thompson using a silenced pistol. They do believe that the shooter likely had a personal vendetta against Thompson or UnitedHealthcare. The bullet cases used in the shooting were marked with the words "deny," "defend," and "depose." The words are similar to a 2010 book titled Delay, Deny, Defend: Why Insurance Companies Don't Pay Claims and What You Can Do About It.

On Thursday, police released a photo of the suspect's face, which was captured by surveillance cameras after an employee at the hostel asked him to lower his mask.

"He lowers the mask and gives that big smile," CNN Chief Law Enforcement and Intelligence Analyst John Miller said. "That little flirtation between the two of them, in some good-humored way, actually yielded what is so far the most significant clue to identifying him."