The sources said Headley, a US national, had done a recce of National Defence College and vital installations in other cities.
Headley, 49, who was arrested last month for allegedly hatching a conspiracy with LeT to carry out strikes inside and outside the US, had allegedly the NDC on his radar. Investigators have said he was also planning a major terror strike in India at LeT's behest.
He was also conducting recruitment of cadres for Pakistan-based terror group Lashker-e-Taiba, to send them to a Gulf country using the services of his immigration business set up in Mumbai.
He was arrested at Chicago's O'Hare International Airport as he prepared to board a flight to Philadelphia, intending to travel to Pakistan.
FBI also arrested his accomplice Tahawwur Hussain Rana, 48, a Canadian citizen of Pakistani origin.
American investigators have said Headley was a frequent flyer, who made multiple trips to Pakistan where he spent "substantial time" undergoing training from the terror group.
Home Minister P Chidambaram has said investigators were probing if Headley had any links with Mumbai attacks as he had visited the country several times before they took place.
"He visited India several times before 26/11 and therefore there is something we have to necessarily look into but as of now I cannot say whether there was a link or not," he told reporters on the sidelines of the India Economic Summit here yesterday.
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