Two packages sent to the Pentagon contained the poison ricin, a Pentagon spokesperson confirmed to Anadolu Agency on Tuesday.
"On Monday, the Pentagon Force Protection Agency detected a suspicious substance during mail screening at the Pentagon's remote screening facility," Col. Rob Manning said in a statement. "The envelopes were taken by the FBI this morning for further analysis."
Manning said all of Monday's mail in the Pentagon's screening facility is under quarantine, but is not a threat to personnel.
Ricin is a poison derived from the seeds of the castor oil plant. It becomes deadly when injected, inhaled or ingested into the body.
It has been used in assassinations in the past due to its toxicity as well as ease in manufacturing.
Bulgarian writer, Georgi Markov, was killed in London from ricin poisoning in 1978.
Source: Anadolu Agency