Utah resident William Clyde Allen, 39, "knowingly threatened to use a biological agent and toxin, specifically ricin, as a weapon," according to a statement from the Justice Department and the US Attorney's Office in the western state.

In addition to Trump, Allen's targets included Defense Secretary Jim Mattis, along with the FBI and CIA directors and commanders of the Air Force and Navy.

Allen was arrested on October 5 shortly after the Pentagon intercepted suspicious mail. He pleaded not guilty.

The ex-soldier also sent Trump a letter containing castor bean material. Ricin naturally exists in seeds of the castor bean, and it is relatively simple to extract the poison from the seeds.

The statement said Allen had purchased 380 castor beans in December 2017.

Ricin is lethal in minute doses if swallowed, inhaled or injected. It is 6,000 times more potent than cyanide, with no known antidote.

Highly toxic, it can cause nausea, vomiting, internal bleeding and difficulty breathing, leading to death from organ failure or the collapse of the circulatory system.

Threatening to weaponize a biological toxin carries a maximum penalty of life in prison, while mailing a threat to the US president can result in up to five years in prison.

The exact content of the threatening messages sent to Trump and top officials were not disclosed, nor were the motives that may have prompted Allen -- a member of the Navy from 1998 to 2002 -- to send them.

His trial is slated for December 26 in Salt Lake City.