The multinational corporation run by billionaire brothers Charles and David Koch has said it will not cooperate with a Senate investigation into whether the corporation has paid for research skeptical of climate change.

Koch Industries Inc., which operates refining, chemical and pipeline companies, was among 100 fossil fuel businesses and organizations that were sent letters by three Democratic senators seeking information on whether they had backed research into global warming.

Koch general counsel Mark V. Holden replied in a March 5 letter to Senators Barbara Boxer, D-Calif., Edward J. Markey, D-Mass., and Sheldon Whitehouse, D-R.I., obtained Tuesday by The Associated Press.

Holden said that disclosing the information would impinge on the Koch corporation’s First Amendment right to participate in public policy debates.

"In reviewing your letter, I did not see any explanation or justification for an official Senate Committee inquiry into activities protected by the First Amendment," Holden wrote.

The Koch brothers are well known for supporting conservative and libertarian think tanks and campaigns through various organizations they fund, including Americans for Prosperity and the Cato Institute. However, it is unclear to what degree they fund scientists who deny climate change.

Last month, documents obtained by Greenpeace through a Freedom of Information Act (FOIA) revealed that the Charles G. Koch Charitable Foundation, a nonprofit organization that funds research and educational programs, had donated money to Wei-Hock Soon of the Harvard-Smithsonian Center for Astrophysics. Soon has long maintained that climate change was a natural phenomenon, not caused by human activities.

Soon told the U.S. Senate Committee on Environment and Public Works in 2003 that the sun, not carbon dioxide, causes global warming — a view that has been widely discredited.

Soon received more than $1.2 million from Exxon Mobile, Southern Company, the American Petroleum Institute and the Charles G. Koch Charitable Foundation to finance his research, Greenpeace said.

“For years, we at Greenpeace have been working to make public the secret paper trails that show what everyone already knows: Climate science deniers — #fakexperts — are few and far between, and most of them are paid by companies most responsible for global warming to downplay the problem,” Greenpeace wrote in a blog post in February.

The Kochs recently announced they would spend nearly $1 billion in the 2016 presidential and congressional races — worrying environmentalists, many of whom say the brothers have a history of backing climate skeptics.

With wire services