WASHINGTON (AP) — Fire and corrections officials in the District of Columbia say over a dozen employees at the city’s jail complex fell ill after a package that likely contained the powerful opioid fentanyl was opened.

The Washington Post reported Thursday that four workers in the mail room were immediately taken to MedStar Washington Hospital Center and three decided to go later.

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Nine others were treated at the jail facility. Officials described the symptoms as relatively minor.

Assistant Fire Chief John Donnelly and D.C. Department of Corrections Director Quincy L. Booth confirmed that the package most likely contained fentanyl. They also confirmed that federal authorities have joined district police and others in an investigation.

Officials say mail deliveries and visits to inmates have been suspended until further notice.