The relationship between the 45-year-old inspector working with the anti-terror wing and Geeta Sharma, a private detective, was not "cordial" and was turning "sour" with the passage of time, Deputy Commissioner of Police (East) Maheshwer Dayal said today.
The two had been living together for the past one year.
After analysing data from the mobiles, laptops and personal and official documents recovered from the flat at Ardee City where the duo was found dead on Saturday, the investigating team in the case suspects that Dutt and Sharma had developed a strained relationship, he said.
Dr Deepak Mathur, who conducted the post-mortem, said both had died almost the same time during the intervening night of Friday-Saturday last week.
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It appears that Dutt used his official weapon of 9 MM bore to fire two rounds, he said.
While one bullet had entered from Dutt's right temple and had exited from the left side, another bullet had entered from the back of Sharma's head, Mathur said, adding that both were killed instantly.
It was "contact range" killing wherein the bullets had been fired by touching the skin of the two, Mathur added.
In contact range killing, the victim does not survive even for a minute.
Sharma was arrested on allegations that she had forged letters of the Union Home Ministry and was released on bail from Tihar jail after spending about three weeks behind bars.
Dutt had joined the Delhi Police in 1991 and was one of the few personnel inducted into the Special Cell in 1994 when the elite unit was formed. He was considered a "master investigator" by his superiors.