House Oversight Committee Chairman Trey Gowdy on Tuesday demanded an explanation about why a scientist associated with the National Cancer Institute failed to use evidence that a widely used herbicide does not cause cancer, thereby allowing an international body to conclude that the herbicide is "probably carcinogenic."

In a letter to the National Institutes of Health, Gowdy noted that an NCI scientist, Aaron Blair, was a senior researcher who reviewed findings from a separate study which said there was no evidence glyphosate causes cancer. Glyphosate is the key chemical in the herbicide Roundup.

But Reuters reported in June that Blair omitted this key research, and Gowdy said that allowed the International Agency for Research on Cancer to conclude in 2015 that glyphosate is a probable carcinogen. Reuters wrote that Blair believed that if the buried research had been included in the IARC study, it probably would have reversed the IARC's conclusion.

"The committee is concerned about these new revelations, especially given Dr. Blair's apparent admission that the AHS study was 'powerful,' and would alter IARC's analysis of glyphosate," Gowdy wrote.

Gowdy's letter asked NIH for documentation related to Blair's decision to keep this separate study, called the Agriculture Health Study, out of the IARC assessment and conclusions.

His request is just the latest attempt by Republicans to uncover how the research was withheld, which has huge implications for Monsanto, which produces the weed killer Roundup. Millions of dollars worth of lawsuits are pending against the company from people who claim they're suffering from cancer because of their contact with the product, and many of the plaintiffs have relied on the IARC findings to bolster their claims.

Last week, Republican Sen. Jim Inhofe sent a letter to the U.S. Department of Health and Human Services asking that the AHS study which shows glyphosate to be non-carcinogenic be made public.

"We have an obligation to make sure that publicly funded studies, like the AHS, are made available as soon as they are complete," Inhofe wrote.

The issue has also led to new scrutiny about the IARC. Last September, the House Oversight Committee wrote to the NIH asking for information on its funding of IARC, and claimed that the federal government had "given several millions of dollars" to the agency since 1992. The agency's controversial findings include declaring things like coffee and meat as cancer causing.

The battles over IARC come amid a political backdrop where President Trump is proposing budget cuts to the National Institutes of Health, which sometimes funds these researchers with grants, and also as congressional Republicans are running legislation they say would provide increased transparency for the science used by federal agencies, but which critics have called a "war on science."