Ricin in a letter to Trump is a poison extracted from castor seeds, which can kill victims in 36 - 72 hours.

US officials discovered earlier last week that the envelope sent to the White House mail processing center contained ricin.

Ricin is a natural substance in the seeds of the castor plant, which occurs in the by-product when the castor seed is prepared into castor oil.

The US Centers for Disease Control and Prevention (CDC) says the possibility of unintentional exposure to ricin is rare, unless castor seed is ingested, while making ricin to poison others is "a yes.

The equivalent of a grain of ricin can also be fatal to humans 36 to 72 hours after exposure, if swallowed, inhaled, or injected.

"Ricin is a poisonous substance. It penetrates inside the cells of the human body and stops cells from making necessary proteins," according to the CDC.

According to the medical nonprofit Mayo Clinic, victims of ricin poisoning have different symptoms, depending on whether they swallow or inhale the poison.

There is currently no antidote for ricin.

In 1978, Georgi Markov, a Bulgarian writer and journalist living in London, died after being attacked by a man holding an umbrella.

In the 1940s, the US military experimented with using ricin as a chemical weapon.

Ricin became popular in popular culture when a character in the American TV series "Breaking Bad" conspired to use it to kill a main character.

The US considers the collection of ricin to be illegal since July 2019.

Some have used ricin in conspiracies targeting President Trump and other officials.

In 2013, James Everett Dutschke, a 42-year-old martial arts teacher, sent two ricin-laced letters to President Barack Obama.

Amesh Adalia, scholar at the Johns Hopkins Center for Health Security, says the lack of an antidote is "the reason some bioterrorism criminals turn to ricin".