Till the agency takes over, the local police would continue with the probe, but a decision on whether the MLA would be arrested would be taken by the CBI, Principal Secretary (Home) Arvind Kumar said.
Sengar, a four-term MLA, enjoys huge clout cutting across party lines in the rural areas around Unnao district, a semi-urban area about 70 km from Lucknow.
According to an official, Sengar is now being interrogated by the CBI sleuths.
There was a nation-wide uproar over the case and the state government recommended the case to the CBI, which detained him on Friay morning, hours after formally taking over.
In the first FIR, the CBI named Sengar and Shashi Singh, a woman who allegedly took the rape victim to the Bharatiya Janata Party leader's house.
A joint team of the CBI and UP cops picked him up from an Indiranagar residence of his kin at around 4:30am.
Singh Sengar and his brother Atul Singh Sengar have been accused of raping a 16-year-old girl last June in a town in his constituency near the state capital, Lucknow. Later, the victim's father was taken into police custody from the location. In a shocking move, the victim's father died in judicial custody with several injuries on his body. The CBI team also met SP Unnao and took her statements on the whole episode. In the interview given to The Indian Express, the victim mentioned the relationship between the two families and those incidents, and he allegedly gang-raped with her a year ago. Since the murder charge was added later by the police, it does not reflect in the CBI FIR. Earlier in the day, the Unnao police, facing heat for its shoddy handling of the case and the recent custodial death of the victim's father, registered a case against Sengar under various sections of the IPC and provisions of the POCSO Act in the Makhi police station area. He also suffered 18 serious wounds as per the post mortem report. The FIRs were filed on Thursday night following the Centre's order. The final report can either be a closure report or a chargesheet.