Maricica Irina Cornici, 23, died last week after spending three days tied to a cross without food or water in the cellar of a church in the north-eastern town of Vaslui as a priest reportedly delivered prayers to drive out evil spirits.
Police found her bound with chains to a cross and a towel stuffed into her mouth.
“We have launched a criminal inquiry against the priest and four nuns on charges of deprivation of freedom followed by the death of the victim,” prosecutor Ovidiu Berinde told Reuters.
Romanian media said the victim, who had been in the monastery for only three months, was a schizophrenic whose behaviour had convinced fellow nuns she was possessed by the devil.
The priest, Daniel Corogeanu, denied any wrongdoing, saying Ms Cornici’s death was the will of God and that she was she was “possessed by the devil”.
When asked whether the nun was mentally ill and in need of medical help instead of exorcism, an unrepentant Corogeanu replied: “You can’t take the devil out of people with pills.”
“God has performed a miracle for her, finally Irina is delivered from evil,” he added.
He insisted that from a religious point of view the crucifixion was “entirely justified”.
But he admitted he faced excommunication as well as prosecution, and was seeking a “good lawyer”.
“Over there, in your world, the people must know that the devil exists. Personally I can find his work in the gestures and speech of possessed people, because man is often weak and lets himself be easily manipulated by the forces of evil,” said the bearded young priest.
“I don’t understand why journalists are making such a fuss about this. Exorcism is a common practice in the heart of the Romanian Orthodox church and my methods are not at all unknown to other priests,” he told the AFP news agency.
Although Romania’s Orthodox Church has suspended the 29-year-old, he has so far refused to stop work at the Holy Trinity monastery.
Bizarrely Father Daniel later celebrated a short liturgy “for the soul of the deceased” in the presence of 13 nuns who reportedly showed no visible emotion.
Claps of thunder from an approaching storm were sometimes the only sounds to break the silence.
“This storm is proof that the will of God has been done,” Father Daniel said.
“You see it?” said the priest, gesturing at the body which still showed the marks of the gag.
It was not clear why Father Daniel believed the nun was possessed.
One parishioner told AFP the nun “had to be punished, she had an argument with the Father during a Sunday mass and insulted him in front of the congregation.”
Vitalie Danciu, the superior of a nearby monastery at Golia, called the crucifixion “inexcusable”, but a spokesman for the Orthodox patriarchate in Bucharest refused to condemn it.
“I don’t know what this young woman did,” Bogdan Teleanu said.