Police and health officials are have released information about poisoned iced tea at a Richardson, Texas restaurant that sickened five people in 2010.
Health officials say the victims are the first known victims of sodium azide poisoning at a public place.
The Centers for Disease Control and Prevention has now disclosed what happened in a nationwide report for doctors.
Authorities said the toxic industrial chemical was deliberately added to a tea urn at the Potbelly sandwich shop when employees weren't looking.
"The ice tea suspicion came up fairly quickly, and so, fortunately, they still had some of that same ice tea batch available that could be tested," said Dr. Paul Wax of UT Southwestern Toxicology.
The local poison control center at the time did not have sodium azide on its watch list as a possible chemical for food poisoning.
Months later, an FBI lab verified that sodium azide was the cause.
Sodium azide has been used as an explosive to set off auto air bags and as a drug to fight hypertension.
The five victims of the April 2010 poisoning have recovered.
No arrests were made.
"Unfortunately, we were unable to determine who may have tampered with the product, and the next step would have been, where did the product come from? And again, that was a process where we were just unable to determine where it originated from," Sgt. Kevin Perlich said.
The Centers for Disease Control and Prevention said the incident is "the first detailed report of sodium azide poisonings at a public venue."
Dr. Kurt Kleinschmidt, a UT-Southwestern Medical Center at Dallas toxicologist who investigated the case, said there have been no related incidents.
Chicago-based Potbelly told The Dallas Morning News that it has now installed tamper-resistant covers on its tea urns.