The case, along with the rape and murder of an eight-year-old girl, has brought Indians onto the streets for mass demonstrations not seen since the rape and murder of a Delhi student in 2012.
The outrage has put pressure on Prime Minister Narendra Modi, whose ruling Bharatiya Janata Party (BJP) is accused of trying to shield its state lawmaker in one case and of defending the accused in the other.
Rape-murder of minor: Protests erupt as victim laid to rest
The Central Bureau of Investigation (CBI) has arrested Kuldeep Singh Sengar, a lawmaker from Uttar Pradesh state which is ruled by the BJP, for allegedly raping the 17-year-old last year.
Police only brought a case against the powerful politician last week after the young victim attempted to burn herself alive outside the state leader's residence.
"We arrested the second person, a woman named Shashi Singh, in our ongoing investigations of the case on Saturday," R.K Gaur, a CBI spokesperson, told AFP on Sunday.
Sengar appeared in court the same day and was ordered detained for seven days.
The girl's family - who fought unsuccessfully for nearly a year to get police to register their case - said Singh had taken their daughter to the state legislator on the pretext of a job.
Singh then allegedly stood guard at the door while Sengar raped the girl, the family's initial complaint to police stated.
Details of the rape and murder of an eight-year-old Muslim girl in Jammu and Kashmir state also made national headlines.
The girl was kidnapped, drugged and repeatedly raped over five days -- including inside a Hindu temple - before being strangled and beaten with a rock.
The killing in the city of Kathua has heightened communal tensions in India's only Muslim-majority state. Scenes last week of Hindu lawyers trying to stop police from entering court to file charges against the eight accused - all Hindus - sparked nationwide revulsion.
Two state ministers from Modi's BJP resigned for attending a rally by local Hindu groups in defence of the accused.
Fourth suspect in gang rape case arrested
The protesters called for a fresh investigation into the case, alleging the real culprits are still at large.
The crimes have shaken India in a way reminiscent of the fatal gang-rape of a Delhi student on a bus in 2012 that made headlines around the world.
Modi on Friday promised justice for the victims as anger mounted, while India's women's minister called for the death penalty for child rapists
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