Eleven people have been taken ill after a letter sent to Fort Myer in Virginia was found to contain a suspicious substance.
Three people were taken to hospital, the fire department said in a tweet.
Two marines were among those affected, ABC News reported.
Authorities on the military base, not far from the Pentagon in Arlington, Virginia, were trying to identify the substance.
The marine corps said in a tweet that the building where the envelope was received was immediately evacuated.