Murder charges were filed Thursday - against unknown persons - 16 hours after two girls were found dead in a field in Uttar Pradesh's Unnao, and after their father gave a statement to the police.
In his statement the father said scarves had been wrapped around the girls' necks when their bodies were found, and that they also had dried foam around their mouths.
Three girls - 13, 16 and 17 years old, and from a Scheduled Caste - left home Wednesday afternoon to gather fodder. When they did not return their family began searching for them.
They were found unconscious in a field and rushed to a hospital, where two were declared dead and the third is receiving treatment - for an as yet unidentified poison - in Kanpur. A hospital bulletin issued this evening said she is in "critical but stable".
Post-mortem reports suggest the girls who died were also poisoned, news agency PTI reported. The post-mortems also revealed no external injuries, the Director-General of Police (DGP) said.
"Postmortem was done by a panel of doctors... no injury before death or external injuries were found. Cause of death could not be ascertained and viscera has been preserved for chemical analysis. We are taking help of forensic experts," DGP HC Awasthi said.
The police have formed six teams to investigate the incident, and have also deployed sniffer dogs - to trace as much evidence as possible - at the area where the bodies were found.
Earlier Unnao Superintendent of Police Anand Kulkarni told PTI that nothing had been ruled out at this stage; this was in response to speculation the deaths were related to honour killings.
Mr Kulkarni, who confirmed the father's statement that dried foam and spittle was found at the crime scene, also said there were contradictions between the mother and brother's statements.
The brother had said the girls' hands and feet had been tied with pieces of cloth, but later retracted that statement; he said he had been told to say that. The mother, Mr Kulkarni said, indicated that the cloth was around their necks.
"We are investigating the matter and questioning people," Mr Kulkarni said.
The family say they had no enmity with anyone.
"We called out their names... but no one answered. Later we found them in our own field. We can't accuse anyone as we did not have any enmity with anyone," a relative said.
The deaths have sparked a political row.
Congress MP Rahul Gandhi accused the Yogi Adityanath government of suppressing Dalit communities and crushing the honour and human rights of women in the state.
BSP chief and former Chief Minister Mayawati offered her condolences to the girls' family and said the incident was "very serious and sad". "The BSP demands a high-level inquiry into the incident and strict punishment for the culprits," she tweeted.
Samajwadi Party MLC Sunil Singh Sajan alleged police in Unnao were trying to hush up the case and sought a probe by an independent agency.
The Chief Minister, PTI reported, had sought a detailed report from the Director-General of Police, and directed hospital officials to provide the best possible care to the surviving girl.
With input from PTI