LEXINGTON, Neb. (AP) — Tyson Foods is investigating what caused an ammonia release that sent 20 workers to a south-central Nebraska hospital.

Firefighters were sent to the Tyson plant in Lexington around 11:15 a.m. Tuesday. The workers had been evacuated by the time firefighters arrived.

Officials say at least one worker was admitted to the Lexington Regional Health Center and one was later transferred to a Lincoln hospital. The rest of the injured workers were treated and released.

Tyson spokesman Worth Sparkman says the plant has been deemed safe, so normal plant operations are expected to resume Wednesday.

He says the ammonia is used for refrigeration.