The investigation comes as peptides grow in popularity, thanks in part to Robert F. Kennedy Jr.’s promotion of the amino acid chains as a way to fight aging and chronic disease. Since becoming Health and Human Services secretary, Kennedy has vowed to end the Food and Drug Administration’s “war on peptides” and other alternative health therapies. Kent Holtorf, the doctor overseeing the booth where the women became ill, also has called for less regulation of alternative therapies and has criticized the FDA for blocking compounds he sees as lifesaving.

Holtorf told ProPublica he is cooperating with the investigation. “Of course, I want to get to the bottom of it. But almost assuredly it will come out that it was not the peptides.”

He said he became convinced the peptides weren’t the cause of the severe reactions after plugging everything he knows about the incident into an artificial intelligence app, which he said gave him a 57-page report that “basically says that it is impossible it was the peptides.” He refused to comment on what the report attributed the illnesses to.