Washington (CNN) Eleven people fell ill after a suspicious letter was opened in an administrative building at Joint Base Fort Myer-Henderson Hall in Arlington, Virginia, on Tuesday, according to the Arlington County Fire Department.

A law enforcement official said field tests for the letter all came back negative for any harmful substance, but the FBI is transporting it tonight to its lab in Quantico for further analysis.

Of the 11 individuals who felt ill, three were transported to a local hospital and are in a stable condition, a spokesperson for the department said.

"An envelope containing an unknown substance was received, today, aboard Joint Base Ft. Myer-Henderson Hall. Personnel in the affected building took immediate preventative measures by evacuating the building," according to Maj. Brian Block, a US Marine Corps spokesperson.

"Base officials and are coordinating with local HAZMAT teams and FBI. Several Marines are receiving medical care as a result of this incident. No additional details are available at this time as the investigation is ongoing," Block said.

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