The actor was found unresponsive in his Malibu home after Los Angeles County Deputies responded to a 911 call on early Thursday morning, according to KNBC. He was pronounced dead at the scene, per TMZ.

The sheriff’s department told the news station that his death was believed to be from natural causes. Madsen’s manager then confirmed this news in an email, stating that his death was the result of cardiac arrest.

“In the last two years Michael Madsen has been doing some incredible work with independent film including upcoming feature films Resurrection Road, Concessions, and Cookbook for Southern Housewives, and was really looking forward to this next chapter in his life,” his managers Susan Ferris and Ron Smith, and publicist Liz Rodriguez, said in a statement.

“Michael was also preparing to release a new book called Tears For My Father: Outlaw Thoughts and Poems,” the statement continued. “Michael Madsen was one of Hollywood’s most iconic actors, who will be missed by many.”

Madsen was perhaps most associated with director Quentin Tarantino thorughout his career. Aside from the Kill Bill series and Reservoir Dogs, he also appeared in The Hateful Eight and Once Upon a Time in Hollywood.