Two suspicious packages were discovered at a Pentagon mail facility on Monday, The Associated Press reports. Then, Tuesday evening, the Secret Service announced that another envelope addressed to President Donald Trump was intercepted before it got inside the White House. Early tests detected ricin in the Pentagon packages, NBC reported. But on Wednesday officials said that the envelopes contained the castor beans that ricin comes from — not the ricin itself, according to the AP.

Whether or not would-be-poisoners are successful at isolating ricin, the poison is still an oddly popular choice for attempting to poison presidents. A man from Mississippi and a woman from Texas, for example, were sentenced to prison for mailing ricin-filled letters to President Obama, among others. President George W. Bush apparently got his own ricin letter, too.

“It’s one of the few poisons that’s actually found in significant quantities in nature.”

Ricin’s popular, in part, because it’s easy to find. The toxin is found in castor beans — the stuff that was apparently in the packages sent to the Pentagon and the White House. They grow on a shrub that’s common in parts of the US. “It’s one of the few poisons that’s actually found in significant quantities in nature,” Peter Chai, a medical toxicologist at Brigham and Women’s Hospital in Boston, tells The Verge in an email.

It’s unusual for people to poison themselves by eating the beans because it requires some serious chewing, and it’s not easily absorbed through the gut, Chai says. The more nefarious way to get at the poison is to isolate it from the mush that’s left behind after pressing the beans for castor oil, which can be used as a laxative.

The poison works by essentially gumming up the protein-building machinery in cells, according to a 2005 review published in JAMA. That typically harms the cells that are dividing quickly, Chai says. And it leads to a whole slew of problems like nausea, puking, bleeding, swelling, plummeting blood pressure, and organ failure, a 2003 paper published in Toxicological Reviews says.

“Ricin is one of the most toxic biological agents known.”

The specific symptoms depend on how a person was exposed to the ricin and how big of a dose they got, according to the CDC. Inhaling it can cause breathing problems, fluid buildup in the lungs, low blood pressure, and death. Eating it can cause vomiting, bloody diarrhea, seizures, organ failure, and, rarely, death, the CDC says. Injecting it can cause weakness, muscle aches, vomiting, fevers, organ failure, and death. (It sounds far-fetched, but ricin injections have happened: BBC journalist and Bulgarian dissident Georgi Markov was murdered by being stabbed with a ricin-tipped umbrella.)

“Ricin is one of the most toxic biological agents known,” according to the CDC. Death can occur anywhere from a day and a half to three days after exposure. And inhaling it (or injecting it) are the more dangerous ways to deliver ricin, Chai says. “Since the toxin is directly absorbed, death is much more likely in those cases.” That could be why assassins like to send it by mail: purified ricin can be made into a powder that goes airborne when the recipient opens an envelope.

Ricin isn’t the only poison lawmakers receive

The really scary thing about ricin is that there’s no antidote, according to the CDC, although there has been work to develop a vaccine. That means the best way to deal with ricin poisoning is to avoid being poisoned in the first place. If someone is exposed, treatment is usually aimed at keeping the patient’s symptoms under control. That means making sure they can breathe, keeping them hydrated, and giving them drugs to prevent seizures and boost their blood pressure.

Ricin is shockingly easy to find – but it can actually be difficult to cook up into a powder that someone might inhale, Erika Check Hayden reported for Nature after the attempted poisonings of President Obama. That may be what we’re seeing with the packages addressed to the Pentagon and the White House — but even mailing a poorly prepared poison is an attempted poisoning nonetheless.

Update October 3rd, 2018 3:15PM ET: Updated to include the latest results indicating that it was not ricin, but a precursor, in the pieces of mail addressed to the Pentagon and the White House.