Sources said Tharoor's domestic help Narayan Singh was interrogated at an undisclosed location in South Delhi. He was interrogated by police at least twice earlier.
Singh, who was in Himachal Pradesh, came to Delhi this morning after investigators asked him last night to join the probe.
His interrogation came a day after a letter, written by Tharoor in November last year to Delhi Police Commissioner B S Bassi, surfaced in which the Congress MP accused investigators of "assaulting" and "intimidating" Singh into "confessing" that they both killed Sunanda.
SIT was formed by the police to probe the case. Pushkar was found dead in a south Delhi hotel's suite on January 17 last year.
Sources said SIT is likely to question Tharoor's personal staff along with the employees of the five-star hotel where she was found dead on January 17 last year, the sources said.