A top US terrorism response centre has discovered it mistakenly used the lethal form of ricin as it trained thousands of emergency responders over the last five years.

The Federal Emergency Management Agency's (Fema) Centre for Domestic Preparedness in Alabama used the lethal form of ricin during training operations for firefighters, paramedics and first responders over the last five years.

A safe form of ricin is supposed to be used for the chemical, radiological, and biological hazards training, Fema spokeswoman Alexa Lopez said.

None of the 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.

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, Lopez said.

About 9,600 first responders trained with chemical or biological agents at the centre's Chemical, Ordinance, Biological, and Radiological department during that time.

"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," 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.

Manager of the vendor Toxin Technology, Bill Rose said all shipments were correctly labelled as the lethal form of ricin and that 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 for students who trained at the centre to submit questions.