WASHINGTON (AFP) – A US man pleaded guilty Monday to illegal possession of ricin, a naturally-occurring poison used in murder and political assassinations and feared as a potential bioterror weapon.

Michael Crooker, 57, of the eastern US state of Massachusetts, also pleaded guilty to threatening a federal prosecutor, the US Department of Justice said in a statement.

Ricin is one of the world's most notorious poisons, killing the victim within three to five days. As little as one milligram could suffice to kill an adult. No antidote currently exists.

Crooker, who has been in custody on various matters since 2004, is scheduled to be sentenced June 20. His plea agreement with prosecutors calls for a 15-year prison sentence, the DOJ said.

Crooker pleaded guilty to one count of possessing ricin without obtaining required registration.

Prosecutors alleged that Crooker admitted in letters to family members that a vial of ricin found at his father's home belonged to him. The father had found the vial by accident.

"The quantity of ricin in the vial was enough to kill 150-750 people," the DOJ said in the statement.

The toxin was used to kill Bulgarian dissident Georgi Markov in London in 1978. He was jabbed with a ricin-filled pellet, just 0.6 millimeters (0.024 of an inch) in diameter, fired from a KGB agent's umbrella.

Ricin is a byproduct of the seeds of the castor oil plant, which is used to manufacture brake fluid, soap, varnish, ink and other products.

As a result, the plant is widely grown, which makes ricin more easily accessible to bioterrorists. It could be delivered in food, water, in a mist or, as in the Markov case, injected.