A top US terrorism response centre mistakenly used the lethal form of ricin as it trained thousands of firefighters, paramedics and other emergency responders during the last five years.
The mix-up happened at the Federal Emergency Management Agency's Centre for Domestic Preparedness, in Anniston, Alabama, which trains first responders to deal with emergencies involving chemical, radiological, and biological hazards.
No students, who wear protective gear in the exercises, were reported to have been harmed, but Fema has asked the Department of Homeland Security's Inspector General to investigate how the toxin came to be used in the exercises.
Fema spokeswoman Alexa Lopez said a form of ricin safe for humans was supposed to be used in the training.
But last month staff discovered an "ongoing discrepancy in the documentation related to the type of ricin being provided". They had ordered the safe version, but it appeared that a supply vendor had been providing the lethal form of ricin to the centre since 2011, she said.
The Anniston Star first reported the potential exposure to trainees.
About 9,600 first responders trained with chemical or biological agents at the centre's Chemical, Ordinance, Biological, and Radiological department during that time.
"However, it's important to note that we have no indication that any students were exposed or harmed. During training, students work with several chemical and biological agents, and use protective measures appropriate for the most dangerous agents," Ms Lopez said.
Ricin, a toxin from the seeds of the castor oil plaint, is lethal to humans in even tiny doses, says Dr William Rushton, an assistant professor and a medical toxicologist in the department of emergency medicine at the University of Alabama Birmingham.
"It's a very dangerous substance. When you get it in your body, it causes all of your organs to shut down," he said.
The toxin has two protein chains - an A-chain that can cause cell damage and a B chain that allows the toxin to penetrate cell membranes. Both chains must be present for ricin to kill, Prof Rushton said.
Ms Lopez said the centre ordered the A-chain-only form of ricin. That form is safe for humans but will still respond to detection equipment as students learn to detect and respond to an emergency involving ricin.
"CDP ordered A-chain, the less toxic strain. All CDP Intended Use Declarations clearly state that we were requesting Ricin Chain A," Ms Lopez said.
USA Today identified the vendor as Toxin Technology of Florida.
Toxin Technology manager Bill Rose said he could not discuss clients but said all shipments were correctly labelled as the lethal form of ricin. He said the company sold only the more lethal form.
"All shipments of products from our company from January 2012 through present have been reviewed and were found to accurately reflect the contents listed on the shipping documents," Mr Rose said.
Fema has set up a web page with information about the situation and students who trained at the centre may also submit questions through a link on that page: cdp.dhs.gov/cdp-use-of-ricin.