Update at 2:45 p.m. — The base’s public affairs office released the following statement Wednesday.

The Joint Base Myer-Henderson Hall Feb. 27 incident where 11 personnel began feeling ill after a letter was opened on the Marine Corps side of the base remains under investigation. NCIS and the FBI are conducting the joint investigation. The three Marines who were transported for additional medical evaluations were released from the hospital at approximately 10 p.m. last night. This office will continue to provide updates as they become available.

Earlier: Firefighters, paramedics and law enforcement responded to Joint Base Myer-Henderson Hall Tuesday afternoon for a hazmat situation involving an unknown substance that was mailed to the base.

Firefighters were first called to Henderson Hall, the headquarters of the U.S. Marine Corps, just after 4:30 p.m. for a hazmat incident. Ft. Myer, Arlington County and Alexandria firefighters and hazmat units were dispatched to the scene, as was 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 victims were Marines.

“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.”

Rep. Don Beyer (D-Va.) said via Twitter that he is “closely following the situation.”

With the help of the local hazmat teams “the building was screened and cleared, and the letter was removed,” the Marine Corps said late Tuesday. The FBI and Naval Criminal Investigative Service (NCIS) are now conducting a joint investigation.

National news media, including major television networks, gathered outside of the base in Arlington’s Foxcroft Heights neighborhood to report on the story. A press conference to be held outside the base was later cancelled. No reason was given for the cancellation.

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

Engine 161’s crew evacuated and decontaminated 11 patients from the hazard area, all evaluated by EMS-3transported to area hospital. All units have picked up, scene turned over to @FBIWFO @FBI pic.twitter.com/cAbNW75zOJ — IAFF LOCAL F253 (@FortMyerFire) February 28, 2018

#Update: Ft Myer Hazmat, 11 people started feeling ill after letter was opened in consolidated admin building. 3 were transported. Condition not known. Ft Myer PIO enroute. — Arlington Fire (@ArlingtonVaFD) February 27, 2018

#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

Closely following the situation at Ft. Myer in Arlington. This is scary, I hope very much that everyone involved will be alright. https://t.co/19WHBmUNCp — Rep. Don Beyer (@RepDonBeyer) February 27, 2018