Police on Thursday registered a case of murder into the deaths of two teenage girls whose bodies were found in a field in Unnao even as a post-mortem revealed no injury marks.
A third girl, who is on ventilator support at a Kanpur hospital, is being treated for suspected poisoning.
The Uttar Pradesh human rights body has taken note of the incident at Babuhara village in Asoha -- around 36 km south of Lucknow -- in Unnao district on Wednesday evening and political parties slammed the Yogi Adityanath government over it.
Although the cause of death could be immediately ascertained in the post-mortem, samples from the girls' viscera have been preserved for further examination, police said.
Police ruled out the suggestion that the victims' hands were tied, citing the absence of any injury marks.
Apart from Section 302 (murder), the first information report also lists Section 201 of the Indian Penal Code, related to causing disappearance of evidence.
The FIR mentioning the two sections was lodged on the basis of the family's complaint.
The three girls, aged 15, 14 and 16, were found in a field by the villagers when they did not return after leaving their house to take fodder for cattle, police had said.
The villagers rushed the teenagers, who are related, to a hospital, where two of them were declared brought dead.
The other girl was rushed to a district hospital and later referred to a Kanpur healthcare facility, according to police.
Chief Minister Yogi Adityanath took a serious note of the incident and sought a detailed report from the director general of police (DGP).
He has directed officials to ensure proper treatment of the teenager, according to a spokesperson.
DGP Hitesh Chandra Awasthi said six police teams had been constituted and senior officials were supervising the probe.
"The post-mortem of the two girls was done by a panel of doctors and no injury before death or external injuries were found on their bodies," Awasthi said in a video clip released to the media.
"The cause of death could not be ascertained and viscera has been preserved for chemical analysis," he said.
"We are taking help of forensic experts and looking into all possibilities."
A medical bulletin by the Kanpur hospital treating the third girl has noted that it was a suspected case of poisoning, the officer said.
Earlier, Unnao Superintendent of Police Anand Kulkarni said there was a contradiction in the statements of the mother and brother of the girls.
He added that the brother had said the girls were tied with a stole, while her mother said on Thursday that it was around her neck.
"Our investigation has found that bodies had no marks on them either in the hands or legs, suggesting that they were not tied, the SP said.
Kulkarni said the family members who were the first ones to reach the spot were being questioned.
Asked if an FIR had been lodged, the SP said the matter was being investigated and action would be taken on the basis of the post mortem report, which was conducted by a panel of three doctors and video-graphed.
On speculation about it being a case of honour killing, the officer said all aspects would be included in the probe, adding that it would not be right to say anything on it at the moment.
Congress leaders Rahul Gandhi and Priyanka Gandhi Vadra hit out at the state government over the issue of women safety.
'The UP government is not only crushing Dalit society, but also the honour and human rights of women,' Rahul Gandhi tweeted.
'But they should remember that I and the entire Congress party will stand as the voice of victims and will endeavour to get them justice.'
He also used the hashtag 'Save_Unnao_Ki_Beti' (daughter of Unnao) in his tweet.
Priyanka Gandhi demanded that the third girl be shifted to Delhi for better medical treatment and asked why her family had been detained.
In a Facebook post, she said: 'The Unnao incident is heart-wrenching. Hearing out the family and providing immediate medical help to the third victim is very important for investigation and justice.'
Bahujan Samaj Party chief Mayawati expressed condolences to the affected family, describing the incident as 'very serious and sad'.
'The BSP demands a high level inquiry into the incident and strict punishment for the culprits,' Mayawati tweeted.
Samajwadi Party supremo Akhilesh Yadav termed it shocking and a 'shame on humanity', and demanded strict punishment for the guilty.
'It seems that the chief minister didn't care about law and order in the state. The state has become insecure for women due to the BJP,' Yadav said in a statement.
He said the Adityanath government had 'failed' to check the atrocities on women.
'The BJP government did not have any policy for women security. BJP ideology ignores women interests. UP has become 'Hatya Pradesh' (murder state) in the present regime,' Yadav added.
Taking suo motu cognisance, the Uttar Pradesh Human Rights Commission sought a report from the SP within two weeks, commission member K P Singh said.
Bhim Army chief Chandra Shekhar Aazad demanded that a post-mortem be conducted by a medical board of AIIMS Delhi and the third girl be shifted there for treatment.
A Samajwadi Party spokesperson said a delegation would visit Unnao on Saturday to meet the family members.
The party's MLC, Sunil Singh Yadav, who is from Unnao, alleged that police was trying to hush up the case and sought a probe by an independent agency.
Congress legislature party leader Aradhana Misra also demanded a judicial inquiry.