NEW YORK (AP) — Investigators are searching for clues that could help them identify the masked gunman who killed the leader of one of the largest U.S. health insurance companies on a Manhattan sidewalk, then disappeared into Central Park. UnitedHealthcare CEO Brian Thompson died in a dawn ambush Wednesday as he walked to the company’s annual investor conference. Security cameras recorded the killing and the initial stages of the gunman’s escape on a bicycle. Police used drones, helicopters and dogs in an intense search, but the killer’s whereabouts remained unknown late into the night. Police had not yet established a motive.