UPDATE
The Powell County Sheriff's Office confirmed on Facebook that all prisoners were accounted for. Montana State Prison issued a press release that said the alarm was sounded due to a "count discrepancy".
The facility is no longer on lockdown, the release said.
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(first report)
The Powell County Sheriff's Office said in a Facebook post that an inmate is unaccounted for at the Montana State Prison.
According to the post, the siren at the prison has been activated, and no further information is available at this time.
This is a developing story. Stay with us for updates as we get them.
