New York (CNN Business) Legendary oil man T. Boone Pickens, whose investments helped shape the American energy industry going back to the 1950s, has died at the age of 91.

Pickens, a long-time Dallas resident, died of natural causes on Wednesday, spokesman Jay Rosser said in a statement on Pickens' website. He had a series of strokes and suffered from head injuries after falling in 2017.

Pickens announced his retirement from the oil and gas industry in January 2018 due to his poor health. In a letter posted to LinkedIn at the time he wrote that "it's no secret the past year has not been good to me" adding that "it's time to start making new plans and setting new priorities."

A longtime maverick in the US energy industry, Pickens began a campaign in 2008 to help the US lessen its dependence on oil from OPEC nations.

In 2007, he had unveiled a proposal for a large wind farm project. The plan failed, mainly due to a steep drop in natural gas prices that made wind and other sources of renewable energy more expensive. "I lost $150 million in the wind business," he complained at a 2015 SALT investor conference.

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