Dr. Paul Fox surrendered his Connecticut medical license in July 2012 after giving up his New York license in May, the Hartford (Conn.) Courant reported Sunday.
New York regulatory records indicate investigators with the Connecticut Department of Public Health alleged Fox had committed "possible boundary violations" with a patient.
The records did not indicate whether the patient was Lanza.
Fox last treated Lanza when he was 15, about five years before a well-armed Lanza entered Sandy Hook Elementary School and shot to death 20 students and six staff members after shooting and killing his mother at home.
Fox, who now lives in New Zealand, was contacted after the Sandy Hook shooting by Connecticut detectives investigating Lanza's mental health history.
He told detectives Lanza was "very rigid ... resistant to engagement" and had "aggression problems."