A law enforcement official confirmed to CNN that the letter "contained derogatory, at time unintelligible and ranting language, and was addressed to a commanding officer at the base".

Police are still investigating if or how the sender, who has not been identified publicly, is affiliated with the base and the officer in question.

According to Specialist Nicholas Hodges who spoke to CNN, a corporal, gunnery sergeant and a colonel were among the sick. However, it is unclear who opened the letter.

Marines and civilians work in the building where the incident occurred.