Authorities in Rhode Island are investigating after two anonymous letters were sent from a state prison threatening to kill three corrections officers and other staff.

Richard Ferruccio, president of the Correction Officers Union, says the letters were found Monday afternoon. They were sent through the interdepartmental mail system at the medium-security facility at the Adult Correctional Institute in Cranston.

Ferruccio says the first letter made specific threats against three officers, included their home addresses and vehicle descriptions and was signed "ISIS."

He says the second letter made threats against a lieutenant and another staff member.

A spokesman for the ACI tells WJAR-TV that the prisoner or prisoners who wrote the letters probably did so because they felt mistreated.

Ferruccio says the ACI and state police are investigating.