Celebrated master whiskey distiller Dave Pickerell passed away in San Francisco on Wednesday evening. He was visiting the city for this year’s WhiskyFest, and died in his hotel room of natural causes, reports say.

Pickerell is best-known for a 14-year stint with Maker’s Mark, as well as the key role he played in founding various whiskey brands including WhistlePig and Blackened. Pickerell also owned his own distilling consultancy firm, Oak View Spirits, which helped hundreds of distilleries launch around the world.

Born in Fairborn, Ohio, Pickerell graduated from the U.S. Military Academy at West Point in 1978 with a BA in Chemistry. He later gained an MA in Chemical Engineering from the University of Louisville, while serving in the U.S. Army in 1988.

Pickerell started his spirits industry career as a distilling consultant, before he was hired as master distiller of Maker’s Mark in 2008. Following 14 years with the Kentucky distillery, Pickerell left to found his consultancy firm and work with a number of startup distilleries and whiskey brands worldwide.

Most recently, Pickerell split his time between four main projects: WhistlePig Whiskey in Shoreham, Vt.; Hillrock in Ancram, N.Y.; the distillery at George Washington’s Mount Vernon plantation in Virginia; and Metallica’s new whiskey brand Blackened.

Earlier today, the band expressed their condolences via Twitter, saying they considered “friend and “partner” Pickerell to be “a member of Metallica.”

“Yesterday we lost a friend, father, and luminary to the world of whiskey,” a WhistlePig spokesperson said. “Our heavy hearts and raised glasses are with Dave’s friends and the entire Pickerell family as we all mourn the loss of a legend.”