Police in New York have released two new images of a "person of interest" in the case of the killing of UnitedHealthcare CEO Brian Thompson.

Posting on social media on Thursday afternoon UK time, the NYPD shared a pair of images of the potential assassin in which he can be seen grinning after the "premeditated attack".

Police have said they're offering a $10,000 reward for any information leading to an arrest and conviction in the bizarre case - just hours after discovering three chilling words inscribed on bullet casings at the scene.