At least two packages containing ricin have been intercepted on the way to the Pentagon.

The existence of the suspected packages has been confirmed by Department of Defence officials, who indicated that the packages never made it into the Pentagon itself. The packages were flagged in the mail delivery building, which is located next door to the Defence Department's massive headquarters.

The FBI has been called in to lead the investigation into the packages, which were addressed to Secretary of Defence James Mattis and to Chief of Naval Operations Admiral John Richardson, according to CNN.

"On Monday, the Pentagon Force Protection Agency detected a suspicious substance during mail screening at the Pentagon's remote screening facility," Colonel Rob Manning said in a statement provided to the media. "The envelopes were taken by the FBI this morning for further analysis. All ... mail received at the Pentagon mail screening facility yesterday is currently under quarantine and poses no threat to Pentagon personnel".

The FBI released the following statement: "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".

As the Washington, DC, area worked to determine the origin of the ricin attack, news also broke that two individuals had been hospitalised after being exposed to a powdery substance in mail sent to Texas Senator Ted Cruz's Houston campaign office.

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The Houston Fire Department responded to that incident, and officials said that the 9th floor was evacuated during the investigation. The two individuals hospitalised, according to the fire department, were not staffers of Mr Cruz's.

"Two people were taken to the hospital after apparently being exposed to a white powdery substance in an office building," the Houston Fire Department tweeted, noting that a HazMat team was responding to the scene and trying to determine the nature of the substance.

It was not clear what the substance delivered to the Cruz campaign office was, or if the mail was at all connected to the ricin mail at the Pentagon.

Ricin can be deadly when purified into a powder, but can take many forms. The substance can be made from castor beans, and is one of the easiest deadly poisons to make.

In June, a Wisconsin woman was accused of spreading Isis propaganda online, and even instructed an individual on the methods of extracting ricin to be used against the government or a water reservoir in the US.

A plan to attack Germany with ricin was stopped by officials in June.