The death of a man found days after an altercation with a Burlington police officer has been linked, in part, to skull fractures he received from the officer punching him while trying to subdue him, according to the man's death certificate.Douglas Kilburn, 54, of Burlington had been the subject of a police call on March 11 when officers responded to the University of Vermont Medical Center for a report of a disorderly individual. Burlington police have said Kilburn punched Officer Cory Campbell in the face, and then Campbell punched Kilburn in return, subduing him. No weapons were involved in the exchange.On Wednesday, Kilburn's death certificate was released showing that his manner of death had been ruled a homicide. His cause of death was "undetermined terminal mechanism due to multiple underlying conditions," including:hypertension,cardiac and cerebral vascular disease,obesity,Type 2 diabetes,and skull fractures due to blunt impact.The death certificate states that the skull fracture he suffered occurred in the parking lot of the medical center on the date and time of the altercation with Campbell. The death certificate says his injury occurred when he was "struck by other person."Kilburn had been treated at the hospital for injuries sustained in the March 11 incident, and he was discharged the next day, Vermont State Police have said. He was found dead three days later.Campbell has been serving on administrative duty since March 18. He was hired by Burlington police on July 19, 2016."He has no disciplinary history and an unblemished record of service," Burlington police said in March.Vermont State Police are continuing to investigate the incident, including whether the officer's use of force was appropriate.