Five Senate Republicans have demanded that Attorney General Loretta Lynch cease the Justice Department's pursuit of climate change skeptics, decrying the use of federal prosecutorial authority to "stifle private debate."
The letter cited several causes of concern: In March, Ms. Lynch testified before the Senate Judiciary Committee that the FBI was looking into information regarding climate change dissent and "whether or not it meets the criteria for what we could take action on."
Last year, a coalition of environmentalists and lawmakers asked the Justice Department to launch a racketeering investigation into ExxonMobil over the company's statements and research on climate change.
In March, 17 attorneys general - 16 Democrats and an independent - announced that they would pursue "fraud" allegations against climate change dissenters. At least two attorneys general - New York's Eric Schneiderman and the Virgin Islands' Claude E. Walker - have issued subpoenas as part of climate change investigations.
"These actions provide disturbing confirmation that government officials at all levels are threatening to wield the sword of law enforcement to silence debate on climate change," said the Wednesday letter from the Senate Republicans.
"As you know, initiating criminal prosecution for a private entity's opinions on climate change is a blatant violation of the First Amendment and an abuse of power that rises to the level of prosecutorial misconduct," said the letter.
The letter was signed by Sens. Mike Lee of Utah, Ted Cruz of Texas, Jeff Sessions of Alabama, David Perdue of Georgia, and David Vitter of Louisiana.
Last week, 13 House Republicans sent letters seeking information from the coalition of state attorneys general, known as AGs United for Clean Power, citing free-speech concerns.
Ms. Lynch's comment at the March 9 oversight hearing came after Sen. Sheldon Whitehouse, Rhode Island Democrat, accused the Justice Department of doing "nothing so far about the climate denial scheme."
"This matter has been discussed. We have received information about it and have referred it to the FBI to consider whether or not it meets the criteria for which we could take action on," Ms. Lynch said.
Mr. Whitehouse has pushed for an investigation into "climate denial" using the Racketeering Influenced and Corrupt Organizations Act, citing the civil prosecution of the tobacco industry undertaken during the Clinton administration.
"The similarities between the mischief of the tobacco industry pretending that the science of tobacco's dangers was unsettled and the fossil fuel industry pretending that the science of carbon emissions' dangers is unsettled has been remarked on widely, particularly by those who study the climate denial apparatus that the fossil fuel industry has erected," Mr. Whitehouse said at the hearing.