An envelope containing an unknown substance was opened at a US military base near Washington, leaving 11 Marines sick and prompting the FBI to launch an investigation.

A gunnery sergeant opened the envelope received yesterday at Joint Base Fort Myer-Henderson Hall in Arlington, Virginia.

People who were exposed reported symptoms including itchy hands and faces and bloody noses, a Marine official told NBC News. The building was evacuated immediately.

A law enforcement official said field tests for the letter all came back negative for any harmful substance, but the FBI is transporting it tonight to its lab in Quantico for further analysis, CNN reported.

The official said the envelope was addressed to the Marines' headquarters battalion on the base, not to an individual.

"Shortly after receiving the letter, 11 people started to feel ill and caused the evacuation of the building," Major Brian Block, a Marine spokesman said.

After the evaluation of 11 people, three personnel were transported in stable condition for further medical evaluations.

The building was screened and cleared.

"The letter was removed, and NCIS (Naval Criminal Investigative Service) and FBI are conducting a joint investigation," he said.

The law enforcement official said the text of the letter contained derogatory, at time unintelligible and ranting language, CNN reported.

Investigators are still determining what relationship, if any, the sender had with the base.

The spokesperson for the Arlington Country Fire Department said that the incident response is being scaled back, however, the investigation remains ongoing.

Fort Myer is adjacent to Arlington National Cemetery and is jointly controlled by the Army and Marine Corps.

It is also home to the Army's Third Infantry Regiment, known as the Old Guard, which serves as the presidential honor guard.