He died at district hospital on Monday after he was shifted under critical condition following April 3 brawl with Senger's brother and his aides.
A Special Investigation Team (SIT) has been formed to probe all allegations related to Unnao matter. said Anand Kumar, additional director general of police (law and order) on Tuesday, hours after the police arrested the brother of Bharatiya Janata Party (BJP) MLA Kuldeep Singh Sengar, who allegedly raped a 18-year-old woman, and three others following the death of the victim's father in police custody.
"Investigation is being done and action will definitely be taken," said Kumar.
He add that the postmortem report states the cause of death was shock and septicemia due to peritonitis and ascending colon perforation.
Atul Singh was arrested from Unnao on the directions of the Uttar Pradesh director general of police (DGP) DGP UP, by the Lucknow crime branch team led by superintendent of police (SP) (crime) Dinesh Singh.
However, the victim claimed that she was unaware of the arrest and urged that the accused be hanged till death.
"Kuldeep Singh (Sengar) isn't being arrested. I don't know if his brother is arrested. I demand that they be hanged till death. They've made my life miserable. I want justice. They killed my father," she told ANI.
Her died at district hospital on Monday after he was shifted under critical condition following April 3 brawl with Senger's brother and his aides.
A defiant Sengar, however, rejected the charge as a conspiracy by his opponents and said the complainant belonged to a "low class", even as six policemen were suspended and chief minister Yogi Adityanath assured whosoever is guilty will not be spared.
The woman's father, aged around 50, was rushed to a hospital from the district jail on Sunday night and died during treatment on Monday, an officials said. He was arrested on April 5 under the Arms Act, they added.
Significantly, a case had been registered against four persons a few days back on a complaint by the deceased's wife that he was beaten up.
"The four accused -- Sonu, Baua, Vinit and Shailu -- named in the FIR registered on April 4, 2018 for beating up the deceased had been arrested," superintendent of police, Unnao, Pushpanjali Devi said.
On Sunday, the woman who has accused the MLA had even tried to immolate herself outside Adityanath's office. Speaking to reporters outside the chief minister's office in Lucknow, Sengar said the allegations against him were part of a conspiracy.
"They belong to a low status family ('nimn star ke log hain')...This is a conspiracy by my opponents," Sengar told reporters, but refused to comment on speculation about he being summoned by the chief minister.
The news of the death spread like wildfire with opposition and the civil society slamming the Adityanath government.
"This is a case of highhandedness because the political power and support that such men like Mr Sengar get from the government, from the CM himself. I think this is something extremely brazen," leading activist Ranjana Kumari said adding that this was a typical case of a politician trying to "bully and bulldoze" people.
"The language he is using is really typical of rooky MLAs," she said demanding a proper inquiry in the woman's allegations.
Under fire, Adityanath sought a report into the custodial death. "The incident is unfortunate. ADG, Lucknow, has been asked to give his report and probe the matter, and if any policeman is found guilty, action will be taken. Whosoever is guilty will not be spared," Adityanath said.
According to UP power minister Shrikant Sharma, a magisterial probe had also been ordered.
The Unnao SP said six policemen, including SHO, Makhi, Ashok Kumar Singh, an outpost in charge and four beat constables were suspended. UP director general of police OP Singh said a Lucknow police team had been constituted to probe the entire matter.
"Whosoever is found guilty, strict action will be taken. None will be spared," he said.
Asked about the allegations against Sengar, the DGP said, "Till now these are allegations. Probe is on. Action will be taken against those found guilty."
District magistrate Ravi Kumar NG said the post-mortem examination of the victim would be conducted by a panel of doctors to ascertain the exact cause of death.
The woman, however, claimed her family had received threats in the past and accused the MLA.
He was "killed at the behest of the BJP MLA" inside the jail, she alleged. "We were getting threats and they killed my father," she told reporters.
The Times of India reported that the family had sent a hand-written letter to the Yogi government, anticipating murder attempt on the jailed father.
"After a fabricated charge, my son has been arrested and sent to jail in a comatose condition. I feel that my family will now be killed by Kuldip Singh (BJP MLA), and his aides Atul Singh and Arun Singh. It is utmost urgent that the police takes legal action against these men," Singh's mother wrote on Sunday.