Several elite U.S. universities are receiving millions of dollars in financial support from fossil fuel companies, new research shows.

According to The Guardian, reports released Wednesday highlight six schools — American University, Columbia University, Cornell University, Princeton University, the University of North Carolina Chapel Hill, and the University of California, San Diego. These reports were put together by student activists working with Campus Climate Network, a global student-led group pushing schools to cut ties with big oil.

To trace fossil fuel funding, the researchers dug into tax records, looked at university boards for members linked to the fossil fuel industry, and checked academic publications for conflict of interest statements, the outlet reported.

The full extent of fossil fuel funding remains murky, as most universities don't disclose their backers. However, a 2023 analysis by Data for Progress estimated fossil fuel companies have funneled at least $700 million to 27 U.S. universities over the past decade.

Some schools are pushing back against calls for transparency. And a 65-page federal report published earlier this year shows companies often attach strings to their grants, requiring academic cooperation.

For example, Princeton University has come under fire for its ties to Petrotiger, a British Virgin Islands-based oil company linked to bribery and corruption scandals. Notably, a number of its former executives were charged with plotting to bribe Colombian officials in order to obtain contracts. Despite Princeton's 2022 pledge to divest from certain fossil fuel companies, the school still received about $140 million from Petrotiger and another $70 million from other fossil fuel firms between 2013 and 2023, the students' research found.

Other reports shared by The Guardian showed that American University had received over $1 million from the fossil fuel sector in the past decade. Additionally, it took in around $1.41 million from the Koch Foundation, a philanthropic foundation owned by the petroleum company Koch Inc., which has a long history of challenging climate research.

Meanwhile, Columbia University has received nearly $44 million in funding from fossil fuel interests since 2005 and has published 784 papers with industry sponsorship, the reports found.