This story has been updated. 

Eleven people, including some Marines, “started to feel ill” after an envelope containing an unknown substance was opened Tuesday at Joint Base Myer-Henderson Hall in Arlington, authorities said.

At around 5:55 p.m., the Arlington Fire Department tweeted that 11 people were sickened after a letter was opened in an administrative building at Fort Myer.

Three people were transported to the hospital, the tweet said, and were in stable condition. A spokesman with the Arlington County Fire Department said firefighters were assisting Fort Myer officials with the incident.

The U.S. Marine Corps said in a tweet that a letter containing an unknown substance was received, and the building where it was opened was evacuated. Several Marines are now getting medical treatment, the tweet said, and the Marines will be coordinating with the FBI to investigate further.

Maj. Brian Block, a Marine spokesman, said in a statement Tuesday night that people in the building where the envelope arrived “took immediate preventative measures by evacuating the building.” U.S. military officials are coordinating with local hazmat teams and the FBI.

“Several Marines are receiving medical care as a result of this incident,” Block said, adding that the investigation is ongoing.

Photographs posted on social media show numerous police cars at the Henderson Hall gate of the base, a complex that includes several small military installations near the Pentagon. Henderson Hall is at the southern edge of Arlington National Cemetery, and used as a headquarters building for the service.

Fort Myer firefighters said they were trying to identify the substance.

Dan Lamothe contributed to this report.