ALTOONA, Pa. — After UnitedHealthcare’s CEO was gunned down on a New York sidewalk, police searched for the masked gunman with dogs, drones and scuba divers. Officers used the city’s muscular surveillance system. Investigators analyzed DNA samples, fingerprints and internet addresses. Police went door to door looking for witnesses. When an arrest came five days later, those sprawling investigative efforts shared credit with an alert civilian’s instincts.