The Pentagon's mail screening facility on Monday had intercepted at least two packages -- addressed to Defense Secretary Jim Mattis and Navy chief Admiral John Richardson -- that authorities suspected to have contained ricin.

"According to our preliminary analysis, the substance was castor seeds, from which ricin is derived," Pentagon spokeswoman Dana White said, noting that the matter remained under FBI investigation.

All mail at the Pentagon was briefly quarantined when the packages were uncovered. No one was hurt.

Separately, the US Secret Service said it had obtained a "suspicious envelope" addressed to President Donald Trump on Monday.

"The envelope was not received at the White House, nor did it ever enter the White House," the Secret Service said.

Produced by processing castor beans, ricin is lethal in minute doses if swallowed, inhaled or injected. It is 6,000 times more potent than cyanide, with no known antidote.