Kuldeep Singh Sengar, a legislator of Uttar Pradesh state from India's ruling BJP, reacts as he leaves a court after he was arrested in connection with the rape of a teenager. File picture: Pawan Kumar/Reuters

New Delhi - Police in northern India on Monday filed a murder case against a ruling party lawmaker accused of raping a teenage girl after a complaint by her family that he had attempted to kill her.

The 19-year-old was travelling in a car with her lawyer and two female relatives in Uttar Pradesh state late Sunday when a truck rammed their vehicle.

The girl and the lawyer were critically injured while her relatives died, police said.

Kuldeep Singh Sengar, a member of the Bharatiya Janata Party (BJP) in the state assembly, was accused of sexually assaulting the girl in 2017. He was arrested has been in jail pending the rape trial since last year.

"We have registered a case of murder, criminal conspiracy against Sengar and nine more people in connection with Sunday's accident, following a complaint from her family," district police chief Sunil Kumar Singh said by phone.

Earlier Monday, the mother of the teenager alleged that Sengar had ordered a hit from inside prison so the rape case does not go to trial.

"It was not an accident but a conspiracy to eliminate all of us," she told reporters.

"Sengar is in jail but his men aren't. He and his men were threatening us ... We want justice."

For most of the day, police had maintained that there appeared to be no foul play and preliminary investigations indicated that it was an accident.

But questions were raised as to why the security detail assigned to the victim was not with her during the time of accident and why the truck's number plate was blacked out with grease.

The BJP, which also rules at federal level, came under attack, with opposition lawmakers accusing the party of shielding the suspect. The Indian parliament was briefly adjourned because of opposition protests over the issue.

"There could be a possibility of murder in this incident; the entire conspiracy should be exposed," said Akhilesh Yadav, former state chief minister Akhilesh Yadav and chief of regional Samajwadi Party.

India's National Commission for Women said it was concerned by the developments in the case. It said it was writing to the state police and would send a team to the state to look into the case.

"History will not judge us kindly if justice is not done to the victim and her family," Priyanka Chaturvedi, a leader from BJP's ally Shiv Sena, wrote on Twitter, urging Premier Narendra Modi and Home Minister Amit Shah to intervene and ensure justice in the case.