BEIJING -- Three prison guards have been put under investigation for suspected negligence and abuse of power after being implicated in a prisoner's fraud racket, the Supreme People's Procuratorate revealed on Tuesday.

The three worked at Nehe Prison in Tsitsihar city, in the northeast province of Heilongjiang.

Suspicions were raised last week when a prisoner, surnamed Wang, was reported to have been chatting and subsequently blackmailing women that lived near the prison, via the cellphone app WeChat.

Wang threatened the women that he would spread videos and pictures that they had sent of themselves naked if they did not visit him in prison. He succeeded in cheating two of the women out of over 110,000 yuan ($about 18,000), according to local prosecuting authorities.

Media reports said Wang owned five mobile phones and had even had sex with one female visitor.

Cellphones are banned in prisons and visiting times are monitored by the prison guards. Wang's case has caused public outcry about lax supervision and corruption in China's prisons.