Three individuals were taken to a local hospital after 11 people became sick after a suspicious was received at a military base in Arlington, Va., on Tuesday, according to the Arlington County Fire Department.

“An envelope containing an unknown substance was received, today, aboard Joint Base Fort Myer-Henderson Hall. Personnel in the affected building took immediate preventative measures by evacuating the building,” U.S. Marine Corps spokesperson Maj. Brian Block said in a statement.

“Base officials and are coordinating with local HAZMAT teams and FBI,” he added. “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.”

According to a spokesperson for the fire department, the three who were hospitalized are in a stable condition and the response to the incident is being cut back as an investigation continues to look into the matter.

Three individuals involved felt a burning sensation on their hands and face, and one experienced a nosebleed, Spc. Nicholas Hodges said.

Whether the letter was opened and who was involved in handling has yet to be reported. Who sent the letter is also unclear.

The incident occurred Tuesday afternoon around 3:30 p.m. at Henderson Hall, a building where Marines and civilians alike work.

The letter was removed and the building was screened and cleared after the incident.