PRINCETON-The cause of high carbon monoxide levels that forced an evacuation Wednesday night at the Gibson County Jail was likely an improperly vented hot water heater, Chief Deputy George Ballard said Thursday.

About 140 inmates were corralled in a recreation area adjacent to the jail and across the street in a small park Wednesday night as police officers armed with shotguns, guarded them.

The inmates were later placed in the under-construction work release center next to the jail.

The building housing the jail was vented by the Princeton Fire Territory for close to five hours, before the carbon monoxide dropped to safe levels.

The last of the inmates were back in the jail by midnight, Ballard said.

Carbon monoxide levels continued to be monitored on Thursday and no further problems were encountered, Ballard reported.

Workers were continuing work to fix the source of the problem on Thursday.