The conglomerate owned by conservative activists Charles and David Koch said it won’t cooperate with Senate Democrats’ investigation into its potential funding of skeptical climate research.

The company’s top lawyer released a letter late Tuesday that he sent last week to Sens. Ed Markey Edward (Ed) John MarkeyDemocratic senator calls for eliminating filibuster, expanding Supreme Court if GOP fills vacancy McConnell says Trump nominee to replace Ginsburg will get Senate vote Massachusetts town clerk resigns after delays to primary vote count MORE (D-Mass.), Barbara Boxer Barbara Levy BoxerThe Hill's Morning Report - Presented by Facebook - Biden, Harris launch Trump offensive in first joint appearance Bottom line Polls show big bounce to Biden ahead of Super Tuesday MORE (D-Calif.) and Sheldon Whitehouse Sheldon WhitehouseLWCF modernization: Restoring the promise Restaurant owner defends calamari as 'bipartisan' after Democratic convention appearance Warren calls on McConnell to bring Senate back to address Postal Service MORE (D-R.I.), invoking the First Amendment to fight the probe.

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He further accused the senators of trying to infringe upon the Kochs' free speech and free association rights.

“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,” Koch General Counsel Mark Holden wrote in the letter.

He added that the senators did not justify why they would investigate activities protected by the First Amendment’s rights of free speech and free association.

Holden said Koch officials “object to your apparent efforts to infringe upon and potentially stifle fundamental First Amendment activities.”

The Koch brothers are very active in conservative political circles and have pledged to spend nearly $1 billion in the 2016 election cycle to support free-market Republicans.

They have been critical of efforts to fight climate change and greenhouse gases and have donated groups that challenge climate change science, including the American Energy Alliance and the Cato Institute.

Koch, which owns subsidiaries in industries like oil refining, agricultural supply and paper products, was one of 100 companies that Markey, Boxer and Whitehouse are investigating.

The senators launched their probe shortly after climate researcher Willie Soon was accused of accepting more than $1.2 million from oil, gas and coal companies in exchange for his research questioning the role of greenhouse gases in climate change.

Activist group Greenpeace said it has documented that Soon received $230,000 from the Charles G. Koch Charitable Foundation.

A similar inquiry by House Democrats led a wide variety of academics and some Republicans to accuse lawmakers of trying to silence skeptical research.