A teenager who accused a prominent local politician of rape in India has been involved in a fatal car crash, with the family and prominent opposition leaders calling foul play.

The 19-year-old alleged rape victim was travelling with her lawyer and two female relatives when their car was rammed by a truck in a head-on collision.

The two female relatives, one of whom was also a witness in a related case, died in the crash, while the alleged rape victim and her lawyer are in critical condition in hospital.

The woman, from Unnao in Uttar Pradesh state, says she was raped in 2017 by Kuldeep Singh Sengar, a member of the state’s legislative assembly for Narendra Modi’s ruling BJP party.

The case came to prominence in April 2018 when the woman attempted to set herself on fire outside the residence of Yogi Adityanath, the BJP chief minister of the state, accusing police of failing to act. A day later, her father – who had brought the case to police on her behalf – died in hospital after he was beaten, allegedly by a group of men including Mr Sengar’s brother.

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The woman’s family said they believed Mr Sengar, who is in prison awaiting trial for the rape charge, was behind Sunday afternoon’s crash.

“I have got to know that [Mr Sengar] got it done,” her mother told reporters, according to NDTV. “[He] is in jail but his men aren’t. He and his men were threatening us ... We want justice.”

A cousin in Unnao said that “everyone in the village knew” the woman and her lawyer would be travelling that day to Rae Bareli prison to visit her uncle, who was jailed shortly after the rape allegations came to light and convicted of an unrelated attempted murder charge.

“MLA Kuldeep Singh and his associates planned the attack to threaten us so that we stop pursuing the case,” the cousin said.

Swati Maliwal, chair of the Delhi Commission for Women, met the alleged victim and her lawyer in hospital in Lucknow, the state capital, and said they were both in life-threatening condition. She shared an open letter to Mr Adityanath, the chief minister, saying she was “deeply disturbed” by the case.

Officially, local police are treating the crash as an accident. OP Singh, director general of police in the state, said a “fair and free probe” would be conducted but that “primary probe suggests it was purely an accident due to an over-speeding truck”. The driver and truck owner had been arrested and were being questioned, he said.

Singh said police would “hand over the case to CBI” (India’s Central Bureau of Investigations) if the family demanded.

It would not be the first time the body, India’s equivalent of the US’s FBI, has had to step in in the Unnao case.

After a public uproar, the CBI was ordered to carry out an extensive investigation and filed three charge sheets last year; against Mr Sengar for the alleged rape, Mr Sengar’s brother and associates for the alleged murder of the woman’s father, and Mr Sengar and others for framing the father on trumped-up charges.

Back in April, after an eyewitness to her father’s beating died in mysterious circumstances, the alleged rape victim told the Times of India she and her family lived in fear for their lives.

“I have told the CBI that I don’t feel safe. I fear for my life – being abducted or killed,” she said.

The woman was living in hiding and had been given round-the-clock police protection. On Sunday afternoon, however, she was travelling without an escort. Mr Singh, the police chief, told ANI this was due to “lack of space in her vehicle” and at the woman’s own request.

Priyanka Gandhi, general secretary of the main opposition Congress party, said Sunday’s crash was “shocking” and that serious questions remained about the case, including why the BJP had failed to expel or even suspend Mr Sengar. “Without answers to these questions, can we expect justice from the BJP government?” she asked.

Rahul Gandhi, the former Congress president, said the case sent the message to Indian women – “don’t ask questions if a BJP MLA is accused of having raped you”.

Arvind Kejriwal, leader of the AAP party, wrote on Twitter: “Systematic attempts to eliminate the Unnao rape victim and her family appear to be a pre-planned conspiracy and a mockery of rule of law. How can any civilised society allow such state sponsored barbarism?”

And Akhilesh Yadav, leader of Uttar Pradesh’s main opposition Samajwadi Party, told the Hindustan Times the crash “could be a murder attempt”.