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(Image: GETTY)

Reports say the entire ninth floor of the Phoenix Tower, in Houston, Texas, was evacuated on Tuesday.

Experts have been deployed to the scene in an attempt to determine the nature of the substance.

The Houston Fire Department wrote on Twitter: "Two people were taken to hospital after apparently being exposed to a white powdery substance...

"The 9th floor of the Pheonix Tower has been evacuated as HFD Hazmat is responding to the scene working to determine the nature of the substance."

(Image: GETTY)

The panic comes just as the Pentagon was put on high alert after a package containing deadly ricin was mailed to the building.

The FBI has launched a probe after the worrying discovery in two suspect packages.

Ricin is a highly toxic compound that is extracted from castor beans and has been used in terror plots.