Two letters addressed to Defense Secretary James Mattis and Chief of Naval Operations Admiral John Richardson were intercepted Oct. 2, along with a suspicious letter addressed to President Donald Trump. All three were laced with ricin.

After being discovered, all U.S. Postal Service letters sent to the Pentagon were put under quarantine. The Central Processing Center (CPC) is located outside the Pentagon. The toxic letters never entered the Pentagon or reached their recipients.

Ricin can be an extremely deadly white substance. In normal cases, it is used for experimentation in killing cancer cells. Ricin can be made from the waste material left over from processing castor beans, in the form of a powder, a mist or a pellet.

If the beans are chewed and swallowed, the released ricin can cause serious injury. Inhaling the powder can cause severe breathing problems and can lead to fluid buildup in the lungs. In the case of inhalation, if left untreated, can lead to low blood pressure and lung failure, causing death.

This is not the first time poison has been sent to political figures. In 2001, letters laced with anthrax spores were arriving at media companies and congress offices just seven days after the Sept. 11 terrorist attack. In the following months, there were five fatalities and 17 other individuals affected.

In 2003, two letters containing ricin were mailed to the White House, one explaining that they wanted trucking regulation hours to stay the same. The second threatened to turn D.C. into a “ghost town”.