John Hillerman reportedly died from heart disease.
The late actor, who portrayed Jonathan Higgins in the long-running TV series 'Magnum, P.I.', passed away last month at his home in Houston, Texas, and it has now been revealed by a spokesperson at the Harris County Institute of Forensic Sciences in Texas that his cause of death was hypertensive atherosclerotic cardiovascular disease.
The disease is caused by high blood pressure, which puts added force against the artery walls and can damage them, making them more vulnerable to plaque build up. This build up of plaque then causes atherosclerosis, and can lead to coronary artery disease if untreated.
The spokesperson told TMZ.com that John also had complications due to chronic obstructive pulmonary disease - which is a type of obstructive lung disease characterised by long-term breathing problems - as well as congestive heart failure, which refers to the build up of fluid around the heart.
John was best known for his 'Magnum P.I.' role of Higgins, a foil to Tom Selleck's titular character Thomas Magnum, a private investigator living in Hawaii.
In 1987, John was handed an Emmy Award for Outstanding Supporting Actor in a Drama Series, and also a Golden Globe gong in the same year for his portrayal of the snooty, but likeable, character.
He received a total of five Golden Globe nominations for his part in the series, which aired on US TV network CBS from 1980 to 1988.
John also starred in long-running CBS sitcom 'One Day at a Time', 'The Betty White Show', and appeared in episodes of 'Hawaii Five-O' in 1976 and 'Murder, She Wrote' in 1992, before retiring from acting in 1999.
The veteran actor made his big screen debut in Peter Bogdanovich's 'The Last Picture Show' in 1971, and went on to feature in several big films, including Clint Eastwood's 'High Plains Drifter' and Mel Brooks' 'Blazing Saddles'.