The domestic help, Narayan Singh, who was in Himachal Pradesh, came here this morning after investigators asked him last night to join the high-profile probe. His interrogattion started around noon and it continued till late in the evening at an undisclosed location in South Delhi.
He was interrogated by police at least twice earlier.
Asked whether Tharoor is a suspect, Delhi Police Commissioner B S Bassi did not give a direct reply saying, "As far as we are concerned anything and everything is possible."
Sources said Singh was asked about details of the people with whom she was apparently in touch on email and different social media including twitter.
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Singh's interrogation came a day after a letter, written by Tharoor in November last year to the Delhi Police Commissioner, surfaced in which the Congress MP accused investigators of "assaulting" and "intimidating" Singh into "confessing" that they both killed Sunanda.
Sources said SIT is likely to question Tharoor's personal staff along with the employees of the five-star hotel in Delhi where she was found dead on January 17 last year.
The sources said Singh was asked whether Sunanda was suffering from any ailments and was there any medication she was taking. He was specifically asked about the Tablet Alprax, two used strips of which were found in her room in Leela Palace hotel.
"He was also asked about the 15 injuries found on her body, especially 'injury number 10' which was termed as a mark caused by the needle of a syringe," the sources said.