David Koch, one of the United States’ richest men and a longtime political power player, has passed away at 79 years old.

“It is with a heavy heart that I announce the passing of my brother David,” Charles Koch, David’s brother, said in a statement on Friday. “Anyone who worked with David surely experienced his giant personality and passion for life. We can all be grateful that it did, because he was able to touch so many more lives as a result.”

The Koch brothers co-owned Koch Industries, an energy and chemical conglomerate, since 1983.

The Koch brothers assembled a vast political network of conservative donors and organizations in support of libertarian-leaning policies and notably founded the small-government group Americans for Prosperity.

“I was taught from a young age that involvement in the public discourse is a civic duty,” David Koch wrote in an opinion-editorial for the New York Post in 2012. “Each of us has a right— indeed, a responsibility, at times — to make his or her views known to the larger community in order to better form it as a whole. While we may not always get what we want, the exchange of ideas betters the nation in the process.”

The Koch brothers have long been controversial names in politics — first as boogeymen to the left for their support of the Tea Party movement during the Obama administration, and now opposing populist conservative policies with advocacy of globalist trade and immigration policies. The billionaire donors, though supporters of many Republican political candidates over the years, did not support President Donald Trump’s campaign for the White House in 2016.

Last year, David Koch stepped away from overseeing the Koch organization and other political networks due to deteriorating health. In a company-wide memo at the time, Charles Koch praised his brother for his “guidance and loyalty, especially in our most troubled times, has been unwavering.”

David Koch was diagnosed with prostate cancer more than two decades ago and had donated millions to cancer initiatives. He served as a board member of the Prostate Cancer Foundation. In recent years, The David H. Koch Charitable Foundation gave money to various arts and science organizations, including the American Ballet Theatre and the Metropolitan Museum of Art.

David Koch received a bachelor’s degree in 1962 and a master’s degree in 1963 from the Massachusetts Institute of Technology (MIT) and joined Koch Industries in 1970. He also served as the CEO of Koch Chemical Technology Group, LLC, a subsidiary owned by the billionaire brothers.

Charles and David Koch each have a net worth estimated at $50.5 billion and tied for fourth place in 2012 on the Forbes 500 list.