Two suspicious envelopes which are thought to contain the deadly poison were sent to Secretary of Defence James Mattis and to Chief of Naval Operations, Admiral John Richardson, according to a US defence official.
The official told CNN the envelopes had initially tested positive for ricin, a highly toxic compound made from castor beans which has been used in terror plots.
The mail facility is in a separate building on the grounds of the Pentagon and the mail never entered the building after being intercepted.
Pentagon spokesman Colonel Rob Manning said in a statement: “On Monday, the Pentagon Force Protection Agency detected a suspicious substance during mail screening at the Pentagon’s remote screening facility.
“The envelopes were taken by the FBI this morning for further analysis.”
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The FBI issued a statement saying it has taken the envelopes for testing.
The statement said: “On Tuesday, October, 2, 2018, in coordination with the Pentagon Force Protection Agency, FBI Special Agents took possession of two suspicious envelopes that had been screened at the Pentagon mail facility.
“Those envelopes are currently undergoing further testing.
“As this is ongoing, we will have no further comment.”
Ricin can come as a powder, pellet, mist or acid.
If ingested it causes nausea, vomiting, internal bleeding of the stomach and intestines before organ failure and death by collapse of the circulatory system.
It comes after multiple people were hospitalised after a “white powdery substance” was sent to the campaign office of US Senator Ted Cruz, according to the Houston Police Department.
The Houston Fire Department said on Twitter: “Two people were taken to the hospital after apparently being exposed to a white powdery substance in an office building at 3200 SW Fwy.
“The 9th floor of the Phoenix Tower has been evacuated as HFD HazMat is responding to the scene working to determine the nature of the substance.”
Tests later determined the substance was negative for any hazardous substance, the Fire Department added.
Offices on the ninth floor of the Phoenix Tower were temporarily evacuated.
The Houston FBI office is investigating.