Sunanda Pushkar was found dead in a luxury hotel room on January 17, 2014 in Delhi.
Sunanda Pushkar's son Shiv Menon was questioned on Thursday in connection with her murder, police said.
Mr Menon, who reached the Vasant Vihar office of the Special Investigation Team (SIT) at 1.15 pm, was questioned for around eight hours till 9.30 pm.
Earlier, Mr Menon, Pushkar's son from an earlier marriage, met the SIT members, who asked him to join the probe.
Sources said he was questioned on many topics related to his mother's relations with her husband, Congress MP Shashi Tharoor.
"Menon was also asked to describe the sequence of events. Why did she shift to the Leela Palace hotel? What happened there? How and when did he come to know about her death? Had his mother shared anything to him before her death? Was she afraid of her life?" a source said.
Mr Menon, who was the first to be questioned by the SIT at such length, reportedly gave some crucial inputs to police which may help them solve the mystery of Pushkar's death.
Mr Menon, who lives abroad, was asked to join the probe as soon as possible.
Sunanda Pushkar was found dead in a luxury hotel room on January 17, 2014. A murder case was registered against unknown people on January 1, 2015.
Earlier, Delhi Police Commissioner BS Bassi indicated that Mr Tharoor and his domestic help Narayan Singh will again be questioned.
"Police will again question Shashi Tharoor and his domestic help Narayan Singh within one or two days," he said.