The Mountain Institution in Agassiz remains on lockdown this morning.
Five staff members at the medium-security facility were sent to hospital after being exposed to an unknown substance on July 26.
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All staff members are OK.
Global News has learned substance may have been fentanyl but the Correctional Service has yet to confirm this information.
Officials say they expect everything to go back to normal once the situation is deemed safe. However, they have not provided a timeline on when that might happen.
Visits have been suspended until the search is completed.