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US businessman T Boone Pickens, a Texas energy investor known for his folksy speech and ruthless business practices, has died at age 91.

Mr Pickens, whose activism on behalf of shareholders was legendary, closed his BP Capital energy hedge fund last year, citing declining health.

He had previously led Mesa Petroleum, an independent oil and gas company, for more than 30 years.

His death from natural causes was announced by his spokesman.

Mr Pickens was a well-known business personality. An early practitioner of activist investing, he launched campaigns against established oil companies and others that made him a household name in the 1980s.

He embarked on takeover bids for larger firms, including Phillips Petroleum, Gulf Oil Corp and Unocal Corp., charging that their leaders had grown complacent.

The efforts failed but succeeded in remaking the industry, pushing Gulf Oil into a multi-billion dollar merger with Chevron - and making him millions along the way.

In 2008 Forbes estimated his fortune at more than $3bn.

Mr Pickens later directed his energies beyond the business world.

He backed the attack ads by the Swift Boat Veterans for Truth group that challenged Democratic candidate John Kerry's military service in the 2004 presidential election.

In 2008, he again blanketed television with advertisements, this time promoting wind energy as part of a $100m effort, which he funded himself, aimed at reducing America's dependence on foreign oil.

He was also a generous donor to Oklahoma State University, his alma mater.

"I like making money. I like giving money away. Giving money is not as fun as making it, but it's a close second," .

Mr Pickens was born in 1928 in Holdenville, Oklahoma. His family moved to Texas in the late 1930s.

Mr Pickens earned a geology degree from what would become Oklahoma State University in 1951 and entered the oil industry, working for Phillips Petroleum for about three years.

In 1954, he struck out on his own. He borrowed $2,500 and with two investors formed an oil and gas firm that would become Mesa Petroleum, which at one time ranked among the world's largest independent oil companies.

Mr Pickens wrote multiple books, including "The First Billion is the Hardest" and maintained an active Twitter presence until just days before his death, where he shared articles and sometimes his so-called Boone-isms, such as, "A fool with a plan can beat a genius with no plan."