Former Columbia County Jail Commander Cpt. Tony Weaver was fired last week.
Sheriff Brian Pixley confirmed Weaver's dismissal earlier this week.
Weaver and Sgt. Carolyn Townsend were both placed on administrative leave in late October, just days apart.
Weaver was suspended on Oct. 24, just one day after Townsend. Pixley said the investigations were unrelated.
The investigation lasted nearly three months and was carried out internally by Steve Salle, a former St. Helens Police Department chief and, more recently, chief deputy for the Sheriff's Office following the abrupt resignation of former Sheriff Jeff Dickerson in 2018.
Pixley declined to provide any details about the substance of the investigation but said it was "nothing concerning on-duty conduct."
Since Weaver's suspension, Lt. Brooke McDowall has served as interim jail commander. Pixley said McDowall will continue in the position but applications for the permanent position are open until Feb. 26.
The chronically under-staffed jail is currently operating with at least four to five fewer staff than needed.
Townsend retired a few weeks after the investigation opened.
The state Department of Public Safety Standards and Training, which certifies public safety officers, will continue to investigate Weaver and Townsend.
DPSST opens investigations into any law enforcement official who is fired or resigns amid an investigation, and ultimately determines whether to revoke the official's credentials.
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