Headley, who has confessed his role in plotting the 2008 Mumbai attack with Lashkar-e-Taiba leaders and was arrested last year in the US, told a team of Indian interrogators in a US prison that he was in the Indian capital in March 2009, said sources who did not want to be identified.
During his Delhi trip -- four months after 10 terrorists sneaked into Mumbai from the sea and killed 166 people over three days -- Headley videographed 7 Race Course Road, the prime minister's official residence, Raksha Bhavan and the National Defence College (NDC) in the heart of the capital, the sources disclosed.
A four-member team of Indian investigators interrogated Headley in June this year after he entered into a plea bargain with the US government offering to be available to foreign investigators for any questioning related to terror plots he was scheming with the LeT and Al Qaeda.
The 49-year-old Pakistani-born revealed that he found "minimal security" at the NDC that appeared a "vulnerable target".
The security cover at the prime minister's residence appeared too tough to break through and his Pakistani handlers were not interested in striking at the Raksha Bhavan, an office-cum-residential complex for defence personnel, Headley is believed to have told the investigators.
He later spoke to his co-conspirator, Tahawwur Hussain Rana, another Pakistani terrorist who is also in a US prison.
He said his Pakistani handlers were more interested in attacking the NDC and had even started working on the idea with a help from an unknown person in Nepal, the sources said.