The Crime Branch of Delhi Police is questioning Apoorva, the wife of Rohit Shekhar Tiwari, who was found dead on April 16 under mysterious circumstances.
The questioning is taking place at Rohit Tiwari's home in Defence Colony in south Delhi. Women security guards are also present inside the house.
On Friday, the Delhi Police had registered a murder case in connection with the death of Rohit, the son of former Uttar Pradesh chief minister and Congress stalwart ND Tiwari.
Rohit Tiwari was declared brought dead to the Max Hospital in Saket on April 16.
The post-mortem report revealed "unnatural death" for Rohit Tiwari and sources in Delhi police said he was most probably killed by being smothered with a pillow. Earlier, the 40-year-old was said to have died due to a cardiac arrest.
Police sources said there are seven CCTV cameras at his home - of which two don't work. They said Rohit Tiwari had gone to Uttarakhand to vote on April 12 and returned on the night of April 15. He could be seen on CCTV taking support of the wall while walking in an inebriated state, police sources added.
The next day, Rohit's mother Ujjwala Tiwari was in Max Hospital for her treatment when she received a call from home that he was unwell and bleeding from his nose. She then took an ambulance that brought him to the hospital.
At that time of the phone call, Rohit Tiwari's wife Apoorva, his cousin Siddharth and domestic helps were at home, police sources said.
Rohit Shekhar Tiwari had fought a six-year-long battle in court to prove that he was ND Tiwari's son.
ND Tiwari had refused to give his blood sample for a DNA test in 2012. He, however, relented later.
In 2014, the Delhi High Court declared ND Tiwari as the biological father of Rohit Shekhar, after the senior leader accepted that he was indeed his son.
The court also restrained Mr Tiwari from denying in public that Mr Shekhar was not his son.
The same year, ND Tiwari, aged 88, married Rohit Shekhar Tiwari's mother.
(With inputs From ANI)