Billionaire David Koch, a major donor to the Republican party, has died aged 79, according to his brother.
The former majority owner of Koch Industries was ranked by Forbes below brother Charles as the 11th richest person in the world.
The native of Wichita, Kansas, ran for president as a Libertarian in 1980, calling for the abolishment of several federal government agencies.
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The brothers had a huge influence on the conservative movement.
Their push for small government and a free economy made them both famous and controversial.
They fell out with President Donald Trump over his trade tariff policy.
In 2018 Koch stepped down from Koch Industries - which refines crude oil, produces fertiliser, and manufactures household products - citing his declining health.
He had been a major donor to the arts, especially ballet, and donated to New York's Lincoln Center and Memorial Sloan Kettering Cancer Center hospital.
According to Forbes, he is estimated to be worth $42.4bn (£34.7bn).
The news of his death was confirmed in a statement from Charles Koch, who said that his brother was first diagnosed with prostate cancer 27 years ago and given only a few years to live.
"David liked to say that a combination of brilliant doctors, state-of-the-art medications and his own stubbornness kept the cancer at bay," Charles Koch wrote, adding that in those years he married his wife Julia Flesher and had three children.
He also described David as having a "giant personality and passion for life" and noted his "institution changing philanthropic commitments to hospitals, cancer research, education and the arts".
Who are the Koch brothers?
The company founded by Charles and David Koch - the second largest privately owned business in the US - has interests ranging from pipelines to paper towels.
According to the Koch Industries website, they have more than 120,000 employees between all their businesses and subsidiaries.
They have previously put money into groups denying climate change and attacking unions and workers' rights.
But they have also pushed for criminal justice reform and made large donations to the American Civil Liberties Union.
