For the plant-based “Impossible Burger” that bleeds like real meat, venture capitalists have forked over millions, and high-profile chefs have called in orders for their swanky eateries. But the Food and Drug Administration, it seems, has chewed it up and spit out safety concerns.

The agency informed Impossible Foods, the company behind the famous faux burger, that it has not proven the safety of the food additive being used to simulate blood and meat-like taste—a protein from the roots of soybean plants called soy leghemoglobin. The protein has not been used in foods before and may be an allergen, the agency said. The concern was revealed in documents obtained by a Freedom of Information Act request filed by environmental and consumer groups, including the ETC Group. The documents were then reviewed by The New York Times.

In a memo that the agency prepared for a phone conversation with Impossible Foods on August 3, 2015, FDA officials wrote:

“FDA believes the arguments presented, individually and collectively, do not establish the safety of soy leghemoglobin for consumption, nor do they point to a general recognition of safety.”

The concern from the agency does not mean that Impossible Foods must stop selling the burger—much to the distaste of consumer groups and critics. In fact, Impossible Foods did not even need to seek the agency’s approval in the first place—it went out of its way to do that.

Under the FDA’s controversial interpretation of the Food, Drug and Cosmetic Act of 1938, the agency created an exemption for such food additives. Food companies can use whatever additives they want as long as they determine internally that the additive is “generally recognized as safe,” or GRAS. And companies do not have to share their safety data with the agency or even notify it of the decision. The FDA only questions decisions and data after the fact, and it then takes appropriate action, if needed.

But Impossible Foods wanted a little extra dash of regulatory pizazz given its growing profile. The company has raised more than $250 million in investments, including some from big names such as Bill Gates. And so far it has served up its burger to high-end restaurants such as Momofuku Nishi in New York and Jardinière in San Francisco.

Biting back

In e-mailed comments to the Times, Impossible Foods spokesperson Rachel Konrad wrote that the company had conducted studies in rats to determine that the additive was safe. She added that an internal panel of experts dubbed the soy leghemoglobin, which is mass-produced using genetically engineered yeast, as safe. The GRAS application Impossible Foods filed with the agency notes that experts considered the soybean leghemoglobin equivalent to other plant and animal hemoglobins commonly consumed.

But, the FDA wanted data showing the ingredient is safe and not an allergen in humans.

Konrad went on to write that the burger “is entirely safe to eat” and “fully compliant with all FDA regulations.” She added that the company is “taking extra steps to provide additional data to the FDA beyond what’s required,” and it plans to resubmit information to the agency this month.

In e-mailed statements to the Times, FDA spokesperson Megan McSeveney confirmed that companies have “no requirement” to prove additive safety with the agency, but McSeveney noted that the agency can take actions later to protect public health.

Jim Thomas, a director at the Canadian ETC Group that initiated the FOIA request, said that the soy leghemoglobin had been “touted as the ‘secret sauce’” of Impossible Foods’ burgers. “Now we know that the FDA had questions about it, but it was put on the market anyway," he told the Times.

In May of this year, a group of other consumer groups, including the Center for Food Safety and the Environmental Defense Fund (EDF), filed a lawsuit against the FDA over its exemption of food additives from pre-market approval.

According to Tom Neltner, a director with the EDF: