The employee was a therapist in the Sex Offender Treatment Program and voluntarily resigned April 11.
Corrections spokeswoman Risa Klemme said employee guidelines do not allow relationships between offenders and staff.
The Spokesman-Review reported that the Airway Heights Police Department is investigating the incident for possible criminal charges.
Officials say the employee could face a charge of first-degree custodial sexual misconduct. Washington law describes that charge as a perpetrator employed by a correctional agency having sexual intercourse with a resident of a prison.
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