Three people were hospitalised Tuesday after they opened an envelope containing an unknown substance at Joint Base Fort Myer-Henderson Hall in Arlington, Virginia.

Eleven people reported feeling ill after the letter was opened, according to the Arlington Fire Department. All three who were transported to a local hospital are in stable condition, the department added on Twitter.

It is unclear what substance was inside the letter, which was opened at an administrative building on the base.

The Marine Corps confirmed base officials are working with local hazmat teams, the FBI and the Naval Criminal Investigative Service (NCIS), adding that the building where the letter was opened has been screened and cleared.

The FBI and NCIS are leading a joint investigation into the incident, the corps said on Twitter

The letter will be sent to the FBI Academy in Quantico, Virginia for testing, ABC News reported.

•Text courtesy of Anadolu Agency.

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