Billionaires Charles and David Koch said they will not cooperate with a Senate-led investigation into whether or not they funded climate-change-denying research, reported Al Jazeera America.

Democratic Sens. Barbara Boxer-Calif., Edward Markey-Mass., and Sheldon Whitehouse-RI sent a letter of inquiry to Koch Industries Inc. requesting documentation concerning their alleged funding of climate research. Koch general counsel Mark Holden replied and denied any cooperation with the Senate investigation.

“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,” said Holden.

Holden also said that the inquiry violated the Kochs’ First Amendment right to engage in public policy debates.

This attempted investigation comes shortly after Greenpeace found that Wei-Hock Soon of the Harvard-Smithsonian Center for Astrophysics accepted money from the Charles G. Koch Charitable Foundation.

In studies funded by that money, Soon asserted that climate change was natural and not man-made. Greenpeace found that Soon has received more than $1.2 million from the fossil-fuel industry to promote bad science. However, the total amount of Koch contributions to scientists is still unclear.

“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,” said Greenpeace.

The Koch money network is massive and far-reaching. The Kochs’ lack of transparency only adds suspicion. With the amount of money they pump into conservative causes, it wouldn’t be surprising to learn they spent large amounts of money on climate-denying research.