Further tests are being conducted on the envelope at another facility for final confirmation, according to Hoback.
No one at the White House screening facility was hurt by exposure to the envelope, according to CNN.
Hoback said the Secret Service would have no further comment because the incident is the subject of an ongoing investigation.
The White House did not immediately respond to an e-mail for comment.
Cyanide is a deadly chemical compound that makes the body unable to use oxygen.
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