Nearly a dozen people, including several Marines, at Joint Base Myer-Henderson Hall fell ill Tuesday after they opened a suspicious letter containing an unknown substance.

The letter was recieved about 3:30 p.m. in Henderson Hall, which Arlington Fire officials described as an administrative building.

A gunnery sergeant opened a letter and showed it to a superior, a Pentagon official told NBC News.

Eleven people began to feel ill, and the building was evacuated. Three Marines were taken to a hospital in stable condition, officials said in a statement.

The Marines were released from a hospital later Tuesday.

The Marines affected reported itchy hands and faces, and bloody noses, a Marine official said. It was unclear how many people suffered from those symptoms.

A large number of police and medics responded to the Northern Virginia military base, which is near the Pentagon and Arlington National Cemetery. The FBI, Naval Criminal Investigative Service (NCIS) and local hazardous material teams cleared the building with base police.

The NCIS and FBI are conducting a joint investigation, a statement said.

Information was not available immediately on what the unknown substance could be.

Stay with News4 for more details on this developing story.