A senior police officer told PTI that based on the post-mortem report, it was assumed to be a case of suicide.
"According to the post-mortem report, the nun died due to drowning. It also indicated the presence of naphthalene balls in her abdomen. Both the wrists were cut, but no other injury marks on her body," he said.
Police would soon submit a report based on the post-mortem report, the officer added.
The body of Sister Susan, a teacher at St Stephens School attached to the convent, was found floating in the well at the sprawling compound on Sunday morning.
Other inmates of the convent had first came across blood stains near the well and then found her body inside it.
Blood stains were also found inside her room.
Local media quoted her relatives as saying that the nun was suffering from some disease and she had been under depression for some time.
Police had registered a case of unnatural death and later shifted the body to the state capital for post mortem.
The death of another Catholic nun, Sister Abhaya, under similar circumstances, had triggered a controversy in the state two decades ago.
Abhaya's body was found in the well of St Pius Convent in Kottayam on March 27, 1992.
She was an inmate of the convent.
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