Security and counterterrorism expert Michael Fagel, who teaches at Northwestern University and is a veteran of ricin investigations, said ricin may be employed because castor beans are so easy to come by.

NPR: 'The Perfect Poison': Ricin Used In 3 Recent Cases

"And you can go on the Internet and find out any one of a gazillion recipes on how to make ricin, " Fagel said, adding that it takes only a beginner's knowledge of science to "weaponize" it.

NPR: 'The Perfect Poison': Ricin Used In 3 Recent Cases