Three people were hospitalized in stable condition after opening a letter containing an unknown substance at Joint Base Myer-Henderson Hall near Arlington, Virginia, authorities said Tuesday.

Eleven people reported feeling ill, Arlington fire officials said. People in the building took "immediate preventative measures" and evacuated the building, the Marine Corps said on Twitter, adding that several Marines were those receiving medical care.

People inside the office where the letter was opened said they began feeling poorly and complained of sore throats, a spokesperson at the military base told The Associated Press.

Officials at the U.S. military base are coordinating with local hazmat teams and FBI investigators.