At least 11 people started feeling ill Tuesday after a letter was opened in an administrative building on Joint Base Myer-Henderson Hall in Virginia.

The letter was received on the Marine Corps side of the base.

Three people were taken to the hospital for evaluation but were released at 10 p.m. Tuesday, according to a command release.

Base officials 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. — U.S. Marines (@USMC) February 27, 2018

Officials did not immediately identify what, if any, substance was in the letter. The FBI and NCIS are investigating the incident.

#Update: Ft Myer, 3 transported patients in stable condition. Command is scaling back incident starting to put some units in service. Investigation ongoing. — Arlington Fire (@ArlingtonVaFD) February 27, 2018

Joint Base Myer-Henderson Hall, just outside Washington, D.C., is the closest installation to the Pentagon and is home to units such as the Army’s 3rd Infantry Regiment, or the Old Guard. It also is where many of the Army’s senior leaders live.

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This is a developing story. It will be updated as we get more information.