As far as poisons go, it's hard to get more lethal than ricin. That's because it takes so little to get a lethal dose.
"A volume roughly the size of a grain of table salt would be a fatal dose for an adult," said Peter Carrington, a toxic plant specialist at Michigan State University. "Something like 1,000 times more toxic than cyanide."
Carrington is familiar with the poison because it's found in the castor bean plant, which is grown in the school's Beal Botanical Garden every year.
Ricin was originally created from what was leftover when castor oil was extracted from the plant's seeds, but now, it's produced as a way to kill.
"This material is so toxic that it's even dangerous for people handling the mail," said Carrington. "It can be produced in a powder or variety of ways."
However, just touching the poison isn't enough to kill. It has to be eaten, injected or inhaled. Once it gets into someone's system, Carrington says it takes hours for the symptoms (chest pains, difficulty breathing, severe coughing) to appear.
"Usually the symptoms are delayed for a while because what we're talking about here is interrupting the protein synthesis of your cells," he said.
That's what makes ricin so deadly. It stops cells from producing the proteins that keep someone alive. Even though it's found in a plant that many use in their landscaping, Carrington says turning ricin into the powder found in letters is not an easy task.
"You have to know some chemistry and have some technique in order to concentrate this stuff into a dangerous and malevolent level," he said.
Ricin is found mostly in the seeds of the castor bean plant, which is recognized by its bright red leaves, but is also found throughout the plant itself. Eating the seeds by themselves can also be deadly.