by ARLnow.com

(Updated at 6:55 p.m.) Firefighters, paramedics and law enforcement have responded to Joint Base Myer-Henderson Hall for a hazmat situation involving an unknown substance that was mailed to the base.

Firefighters were first dispatched to Henderson Hall, the headquarters of the U.S. Marine Corps, just after 4:30 p.m. for a hazmat call. Ft. Myer, Arlington County and Alexandria firefighters and hazmat units are all on scene, as is an “EMS task force” that is usually dispatched to mass casualty incidents.

Initial reports suggest that a certified letter was opened in one of the buildings and that it contained some sort of potentially hazardous substance, prompting an evacuation of the building and the deployment of an emergency decontamination station.

Eleven people were treated for symptoms and three were transported to the hospital in stable condition, according to the Arlington County Fire Department. Symptoms included a nose bleed and a burning sensation, according to initial reports.

A Marine Corps official released a statement saying that the base was coordinating with the FBI.

“An envelope containing an unknown substance was received, today, aboard Joint Base Ft. Myer-Henderson Hall,” the statement said. “Personnel in the affected building took immediate preventative measures by evacuating the building. Base officials are coordinating with local hazmat teams and the FBI. Several Marines are receiving medical care as a result of this incident.”

National news media has gathered outside of the base in Arlington’s Foxcroft Heights neighborhood to report on the story. Rep. Don Beyer (D-Va.), meanwhile, said via Twitter that he is “closely following the situation.”

During the incident police closed off the road near the entrance to Henderson Hall, at the intersection of S. Orme Street and Southgate Road.