Turkeys raised without the use of antibiotics are seen at a farm in Lebanon, Pa. on April 11, 2012. Matt Rourke/AP

The U.S. Food and Drug Administration allowed dozens of antibiotics used in animal feed to stay on the market despite findings by its own researchers that the drugs will likely expose people to antibiotic-resistant infections, according to a report released Monday by an environmental advocacy group.

The FDA reviewed 30 different types of antibiotics that are put into livestock feed and found that 18 of them pose a “high risk” of exposing humans to antibiotic-resistant bacteria through the food supply, the Natural Resources Defense Council said in a report about the FDA’s findings.

The manufacturers of the other 12 drugs didn’t submit enough information to the FDA in order to determine whether they could be safely used in animals without harming humans, but the FDA did not withdraw its approval of any of the antibiotics following its review, according to the NRDC.

“To our knowledge, FDA has taken no action since the reviews to revoke approvals for any of these antibiotic feed additives,” the NRDC’s report said.

“I was shocked,” Carmen Cordova, a microbiologist for the NRDC and lead author of the report, told Al Jazeera.

“These are their very own findings,” she said. “This is still a question we have as to why there’s been no action, essentially, for 40 years. What we think is that they simply bow to industry.”

The NRDC accessed the FDA’s review of the drugs, which the agency conducted from 2001 to 2010, through a Freedom of Information Act request and the settlement of a subsequent lawsuit NRDC filed against the FDA.

The FDA’s documents, which were partially reviewed by Al Jazeera, also found that 26 out of the 30 antibiotics didn’t meet the safety standards laid out by the FDA in 1973 requiring drug manufacturers to submit evidence that their drugs didn’t create antibiotic resistance that could endanger human health.

The FDA’s review did not clearly indicate how many of the drugs continue to be sold today, but the NRDC said it found evidence that at least nine of the antibiotics continue to be marketed as additives to animal feed.

Some of the drugs in the FDA's review have been used since the 1950s, the NRDC said, and while the FDA sent letters to some of the drug companies asking for more safety information, it didn’t take further action and none of the companies reportedly submitted extra safety studies.