Police have concluded their investigation into the death of a Boeing whistleblower who was found dead from an apparent suicide in March, RadarOnline.com has learned. The investigation determined thatlongtime Boeing quality control managerJohn Barnett, 62, died of a self-inflicted gunshot wound, the Associated Press reports. “Information and records reviewed during the investigation uncovered Mr. Barnett’s longstanding mental health challenges, which had intensified in connection with ongoing legal proceedings related to his whistleblower case,” police said in a statement. “We remain acutely aw…