Four Democratic senators are sharply criticizing a conservative think tank’s efforts to bring climate change skepticism into the nation’s public schools as “industry funded” and “possibly fraudulent” and demanding to know whether federal education officials have been in contact with the group.

The Heartland Institute has been sending books, DVDs and pamphlets to science teachers across the country promoting its stance that climate change is caused by natural phenomena rather than human activities — a view rejected by nearly all climate scientists. Heartland’s campaign to influence how climate science is taught in public schools was first reported by FRONTLINE and The GroundTruth Project in March.

The Illinois-based non-profit vows to continue its efforts, saying it has already sent more than 300,000 packages to K-12 and college-level science teachers since the launch of the campaign earlier this year. Mailings have wrapped up for the summer break, according to Heartland spokesman Jim Lakely. He added, however, “This is not the end of Heartland’s efforts to bring balance to the climate debate in our schools, but just the beginning.”

In a letter sent to Education Secretary Betsy DeVos on Wednesday, Sens. Sheldon Whitehouse (D-R.I.), Brian Schatz (D-Hawaii), Edward Markey (D-Mass.) and Elizabeth Warren (D-Mass.) — all of whom have been outspoken on climate change issues — described the campaign as an effort “to disseminate fossil-fuel industry talking points as curriculum for science teachers.”

The senators asked DeVos whether any Education Department officials have had contact with individuals associated with the Heartland Institute “on climate, science, or science education issues,” and whether any informational resources put out by the department have been created in collaboration with Heartland.

The senators also asked DeVos whether she was aware of any discussions between the White House staff and Heartland. In March, the group’s president and CEO, Joseph Bast, told FRONTLINE and GroundTruth, “We’re getting a lot of requests for expert opinion from the White House … That’s very new.”

Heartland was founded in 1984 and has a history of promoting causes that align with the interests of industry. In the 1990s, for instance, the organization lobbied against smoking bans, for which it was rewarded with donations from tobacco company Philip Morris. Today, the group promotes a bevy of libertarian and conservative causes, including one DeVos has spent years advocating for: The school choice movement, which would allow students to spend public dollars to attend private and religious schools.

But Heartland is perhaps best known for its position as a leader in the movement to reject the scientific consensus on global warming. Among the group’s arguments is that curbing greenhouse gases would damage the economy — the primary reason given by President Donald Trump last week for pulling out of the Paris climate accord.

DeVos has, until recently, been relatively quiet on her personal view about climate change. During her nomination process in January, she was asked by Whitehouse whether she believed that humans cause climate change. DeVos answered that because the Education Department has no jurisdiction over climate change, “I would respectfully defer to my colleagues in other agencies” on the issue, and noted that the department “is prohibited from dictating curricula in our nation’s schools.” States and school districts, rather than the Education Department, direct curricula and science standards.

However, last week DeVos issued a statement praising President Donald Trump’s decision to withdraw from the Paris climate accord. “The announcement made today by the President is one more example of his commitment to rolling back the unrealistic and overreaching regulatory actions by the previous Administration,” the statement read.

President Trump making the announcement that the U.S. will withdraw from the Paris Accord.

It was that statement that prompted Whitehouse and his colleagues to write the letter, he said. “It raises at least a suggestion that there may be more mischief going on between the fossil fuel industry and its denial front groups and the administration through Secretary DeVos,” the senator said in an interview.

DeVos did not immediately respond to a request for comment.

Heartland declined to comment on whether anyone at the organization has corresponded or met with staff at the Education Department, but in a written statement Bast said that his organization stands by the materials it has sent to teachers, and will continue to send out copies as it sees fit. Bast said the senators’ letter consisted of “purely partisan rhetoric apparently aimed at undermining the presidency of Donald Trump,” and said that Whitehouse is guilty of the very thing Heartland has frequently been accused of: bringing “politics into high school classrooms.”

The senators’ letter isn’t the first Democratic condemnation of the mailing campaign: In April, three ranking Democrats in the House publicly urged Heartland to stop the mailing campaign

Whitehouse also sent letters to a dozen scientific and educational organizations to “alert” them to “factually inaccurate and scientifically unsound materials on climate change.” He said he worries that the Heartland materials will successfully sway some teachers’ opinions of climate change.

“It takes a bit of sincere effort to do the research and find how fraudulent many of their assertions are, and that’s an effort that busy teachers may not undertake,” he said. “So it leaves at least a passing impression in an innocent teacher’s mind that there may actually be something to this.”