Envelopes sent to Defense Secretary Jim Mattis and Chief of Naval Operations Adm. John Richardson at the Pentagon have initially tested positive for ricin, a defense official told CNN.
Neither men were exposed to the toxin, since all mail to the Pentagon is routed through a mail screening facility that is nearby but separate from the building. Mattis is currently traveling in Europe and will attend a NATO Defense Ministerial in Brussels on Wednesday.
"On Monday, the Pentagon Force Protection Agency detected a suspicious substance during mail screening at the Pentagon's remote screening facility," Army Col. Rob Manning, a Pentagon spokesman, said in a statement to Task & Purpose.
"All USPS mail received at the Pentagon mail screening facility yesterday is currently under quarantine and poses no threat to Pentagon personnel."
The FBI has taken possession of the envelopes and is conducting further analysis, Manning added.
Ricin is a poison that is naturally found in castor beans that can cause injury or death if ingested, according to the Centers for Disease Control.