A Navy veteran has been charged with threatening to use a biological toxin as a weapon by sending letters to US president Donald Trump and other leaders containing ground castor beans.

William Clyde Allen III, 39, told investigators he wanted the letters to “send a message,” though he did not elaborate, FBI investigators said in documents filed in US District Court of Utah.

Allen also said he sent similar letters to Queen Elizabeth II, Russian President Vladimir Putin and the secretary of the Air Force, though it is not clear whether those envelopes had been found.

Suspicious Letters
Allen, a US Navy veteran, was arrested on Wednesday (AP)

Authorities traced Allen after finding his return address on the envelopes which contained the substance from which the poison ricin is derived, according to the complaint.

The envelopes that tested positive for ricin also had a note that said “Jack and the Missile Bean Stock Powder,” the documents said.

US Attorney for Utah John Huber declined to comment on Allen’s mental state, but said the case is “no laughing matter”.

Belgium NATO
Among the recipients of the letters was Jim Mattis (Francisco Seco/AP)

“When you’re dealing with suspected ricin, this is nothing to trifle with,” he said.

During a court hearing on Friday, Allen cried as he told a judge that his wife suffers from a spinal condition and he helps her put on her shoes in the morning.

He smiled at family members and said he had been looking forward to an upcoming general conference of The Church of Jesus Christ of Latter-day Saints. He did not enter a plea, and his lawyer, Lynn Donaldson, did not comment.

Allen could face up to life in prison if convicted on the biological toxin charge, one of five counts in the complaint.

He is also charged with four counts of making threats through the mail, which carry 10-year sentences.

The envelopes were mailed to the president, FBI Director Christopher Wray, Defence Secretary Jim Mattis and the Navy’s top officer, Admiral John Richardson, authorities said.

They were intercepted and no one was hurt.

The FBI said all of the letters tested positive for ricin.

The case is expected to go before a grand jury and Allen could face additional charges at a hearing October 18.

Allen was arrested on Wednesday at his house in the small city of Logan, north of Salt Lake City.

He told investigators he had purchased castor beans on eBay “in case Word War III broke out,” so he could “defend our nation”.

Allen served in the Navy from 1998 to 2002, according to Navy records.

He has a criminal record in Utah including child abuse and attempted aggravated assault.

He also had a history of sending threatening emails over the last few years to then-US president Barack Obama, the Air Force and the state of Utah, investigators said.