The weapon used in the killing of UnitedHealthcare CEO Brian Thompson was identified as the B&T Station Six. This firearm is a modernized version of the British Welrod spy pistol from World War II. Renowned for its discreet design, near-silent operation, and distinctive rotating bolt mechanism, the Station Six combines historical roots with contemporary technology.
Sources:
https://www.capitolarmory.com/bt-station-six-9mm-suppressed-pistol-bt410111.html
https://www.bbc.com/news/live/cwyp1z53w0jt?post=asset%3Aae1573b6-c1c1-4180-b8f1-57341e6e6485