On Monday, Reps. Alexandria Ocasio-Cortez (D-NY) and Chellie Pingree (D-ME) asked the heads of Facebook, Google, and Microsoft to pledge their companies’ support for the science suggesting that climate change has made a significant, negative impact on the environment.

The lawmakers penned the letter after news broke last week that the companies sponsored a conference, LibertyCon, which promoted climate change denialism. At the libertarian-focused conference, a speaker from the CO2 Coalition gave a talk arguing that the environmental impact of climate change has been exaggerated.

“We are encouraged that each of you have pledged to reduce your carbon footprint and have committed other efforts like pursuing renewable energy,” the two lawmakers wrote. “Disappointingly, though, the example you have set promoting sustainability and evidenced-based science is compromised by your implicit support of the session organized at LibertyCon.”

At the conference, the session cited by the lawmakers and reports was the only in support of climate denialism. The conference was not focused solely on environmental issues, but other other sessions were held asking whether carbon taxes were a viable solution for climate change, and religious liberty.

“Given the magnitude and urgency of the climate crisis that we are now facing, we find it imperative to ensure that the climate-related views espoused at LibertyCon do not reflect the values of your companies going forward,” the lawmakers wrote.

In a response to Monday’s letter, Microsoft spokesperson told The Verge, “Microsoft’s engagement with LibertyCon was through a breakfast reception for student leaders to discuss topics including rural broadband and privacy.”

“Climate change is one of the most important issues of our time, and our commitment to addressing it is unequivocal,” the spokesperson said. “For the past decade, we’ve reduced emissions, invested in renewable energy, supported policies to address climate change and enabled people around the globe to use technology to accelerate progress on this issue.”

In a statement to Mother Jones, Google said that it often sponsors organizations across the political spectrum. “Google’s sponsorship or collaboration with a third party organization doesn’t mean that we endorse the organization’s entire agenda or agree with other speakers or sponsors,” a spokesperson said.

Facebook declined to comment.

The letter to the likes of Mark Zuckerberg and Sundar Pichai isn’t Ocasio-Cortez’s first big criticism of Silicon Valley. Over the weekend, she continued to critique companies like Amazon over algorithm bias and platforms like Google and Facebook for disseminating misinformation.

“The fact of the matter is the current monopoly trend is societally and economically unsustainable,” Ocasio-Cortez said on Saturday.

In recent weeks, Ocasio-Cortez, who is known for her efficient and creative uses of social media, has turned her sights onto Silicon Valley tech giants. Shortly before she took her oath of office in January, Amazon announced that its second HQ would be divided between New York City and Northern Virginia. While her New York colleagues in the Senate initially kept quiet about the announcement, Ocasio-Cortez sided with many local leaders, rejecting Amazon’s decision to move into Long Island City.

Updated 1/28/19 at 4:17 p.m. ET: Updated to include statements from Microsoft and Google.