Envelopes containing a suspicious substance have been sent to the Pentagon.
It's feared the two envelopes could contain the potentially deadly poison ricin.
Initial tests are said to have found that the envelopes contained the poison.
Officials at the US defence headquarters say mail received yesterday at its postal facility - which is separate from the main building at the complex - is under quarantine following the discovery on Monday.
The FBI is now analysing the packets, which were reportedly addressed to US Defence Secretary Jim Mattis and US Navy chief Admiral John Richardson.
In a statement quoted by US media, Pentagon spokesperson Col Robert Manning said mail received yesterday "poses no threat to Pentagon personnel" since it was quarantined.
Ricin is a substance found naturally in castor beans, but the US Centers for Disease Control and Prevention (CDC) says it would "take a deliberate act to make ricin and use it to poison people".
CDC explains: "Ricin works by getting inside the cells of a person’s body and preventing the cells from making the proteins they need.
"Without the proteins, cells die. Eventually this is harmful to the whole body, and death may occur."
