The 67-year-old was found unresponsive in his home in Malibu, California, last Thursday and pronounced dead.
His doctor said heart disease and alcoholism will be listed as factors which contributed to the star’s death, reported NBC Los Angeles.
With no suspicious circumstances and the death listed as being from natural causes, the Los Angeles Sheriff’s Department considers the case closed.
In a career spanning more than 40 years, Madsen’s film credits include Free Willy, Donnie Brasco and Sin City.
He was also known for his collaborations with director Quentin Tarantino, including in Kill Bill: Vol. 2, The Hateful Eight and Once Upon A Time In Hollywood.
The Chicago-born actor also linked up with Tarantino when he played Mr Blonde in 1992’s Reservoir Dogs.
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His sister, Oscar-nominated actress Virginia Madsen, paid tribute to her brother in a statement to Variety.
She wrote: “My brother Michael has left the stage.
“He was thunder and velvet. Mischief wrapped in tenderness. A poet disguised as an outlaw. A father, a son, a brother – etched in contradiction, tempered by love that left its mark.”
Madsen was preparing to release a new book called Tears For My Father: Outlaw Thoughts And Poems.
A statement by managers Susan Ferris and Smith, and publicist Liz Rodriguez, said the book by “one of Hollywood’s most iconic actors” was currently being edited.
