Shell casings found at the site where a masked gunman shot and killed United Healthcare CEO Brian Thompson had the words "deny," "defend" and "depose" scrawled on them, a senior New York City law enforcement official briefed on the investigation told NBC News on Thursday. The presence of the words was first reported by ABC News.

Thompson, 50, was killed after being hit by at least three shots outside the New York Hilton Midtown early on Wednesday while on his way to speak at UnitedHealth Group's investor conference. NYPD Commissioner Jessica Tisch said the "brazen, targeted attack" was "premeditated," but police have said the motive remains unknown. The gunman is still at large.

A security video shows Thompson in a blue suit walking down the street before the suspect approaches from behind, firing from his gun. Thompson stumbled forward as a witness fled behind the gunman. The suspect continued to shoot Thompson as he fell to the ground, appearing to clear a jam in his gun between shots.

Former FBI supervisor Rob D’Amico told NBC News that the shooting has the markings of a personal vendetta tied to United Healthcare, which provides health insurance to millions of Americans. Thompson had been receiving threats, but nevertheless did not use a security detail.

“There had been some threats,” Thompson’s wife, Paulette, told NBC News. "I don’t know details. I just know that he said there were some people that had been threatening him.”

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