Agbayani ordered lawyer Basilio Rupisan, provincial legal officer, to conduct the investigation.
The inmates also alleged rampant mulcting by jail guards, victimizing their families and visitors.
Last week, Agbayani designated one of his aides, Benzon Cadiente, as provincial jail warden, replacing retired Col. Alejandro Tamani who was appointed full-time head of the human resource management office of the provincial government.
It was not known if the sudden replacement of Tamani, who had been holding dual positions as jail warden and concurrent human resource management officer for years, had anything to do with the alleged irregularities inside the jail.
Earlier, inmates, who requested anonymity for fear of reprisal, revealed to local media several alleged irregularities inside the jail, including the selling of shabu to inmates.
They also accused jail guards of selling liquor at an exorbitant price. A bottle of gin, sold outside at P20, was being peddled to them at P50, they said.
"Sometimes, they don’t even treat us as humans. They treat us like pigs," an inmate said.
They also complained of so-called "toll fees" being collected from their visitors. Those who paid the amount were reportedly allowed to stay inside the provincial jail even beyond the designated visiting hours.
Visitors who failed to "bribe" the jail guards, however, were frisked and allowed to stay for only a limited period, they added.
At present, there are more than 100 inmates at the provincial jail, each receiving a daily food subsidy of P25.
Recently, the provincial jail was adjudged as this year’s healthiest jail in the region.