The FBI is investigating a suspicious substance mailed to an Arlington, Virginia military base after 11 people, including several Marines, become ill on Tuesday.

The letter was addressed to a commanding officer at the Joint Base Fort Myer-Henderson Hall and was opened in an administrative bloc, law enforcement officials told CNN. The sender has not been named.

After the letter, which was described as irate, and at times unintelligible, was opened, several people reported experiencing a burning sensation on their hands and face, according to Specialist Nicholas Hodges who spoke to CNN.

Local first responder and HAZMAT crews were called to the scene and three Marines were reportedly transported to a local hospital for additional treatment. The three are in a stable condition. Eight others complained of symptoms like nasal irritation and itching, according to ABC.

“Base officials are coordinating with local HAZMAT teams and FBI. 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,” Major Brian Block, a U.S. Marine Corps spokesperson said in a statement.

Initial field tests found that no known toxic substances were linked to the letter, but the FBI is reportedly taking the contents to its Quantico lab for further analysis.

Joint Base Myer-Henderson Hall is an Army and Marine base near the Pentagon that is commonly-known as Fort Myer.

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Write to Laignee Barron at Laignee.Barron@time.com.