The Koch brothers, who head America’s second-largest private company, have refused to provide information requested by Senate Democrats investigating links between scientists who deny the risks posed by anthropogenic climate change and the fossil fuel industry, according to a report by the Associated Press (AP). Koch Industries, which was among 100 companies approached by the senators, reportedly said that the investigation was an attempt to “infringe upon and potentially stifle” their First Amendment rights.

“The activity and efforts about which you inquire, and Koch's involvement, if any, in them, are at the core of the fundamental liberties protected by the First Amendment to the United States Constitution,” Mark V. Holden, Koch’s senior vice president and general counsel, reportedly said in response to the Senate request sent in February.

“To the extent that your letter touches on matters that implicate the First Amendment, I am sure you recognize Koch's right to participate in the debate of important public policy issues and its right of free association,” Holden said, adding that the senators had failed to provide “any explanation or justification” for their inquiry.

Last month, three Senate Democrats -- Ed Markey, Barbara Boxer, and Sheldon Whitehouse -- had launched an investigation into funds provided by fossil fuel companies and organizations to scientists conducting climate-related research.

The probe was launched after The New York Times reported in February that Wei-Hock Soon, a scientist at the Harvard-Smithsonian Center for Astrophysics, and a prominent voice of the so-called “climate skeptics,” had received over $1.2 million from the fossil fuel industry, including at least $230,000 from the Charles G. Koch Charitable Foundation. Moreover, he had failed to disclose the apparent conflict of interest in most of his scientific papers, according to the Times.

Koch Industries, headed by brothers Charles and David Koch, owns companies with interests in oil-refining, petrochemicals, and oil and gas pipelines, and has an annual revenue of over $115 billion. In the past, the brothers have been repeatedly accused of funding climate-change deniers and blocking attempts to curb global warming.