As he led the agency into the Pakistani deep state where the ISI ran the Le T as a strategic asset in its war against India, they discovered that Headley was the agent provocateur who had expanded the scale of 26/11.
"The germ of the idea to attack Mumbai existed," an intelligence official says, "Headley's meticulous inputs gave the ISI and the Le T the confidence to make it much bigger." The Mumbai attack is, to date, the world's only hybrid terror strike that combined all the elements of modern terrorism: Seaborne infiltration, suicide commandos and vehicle-borne explosives.
"Headley was a womaniser and wife-beater who consumed these drugs to keep in shape," writes Zaidi.
To all those he met and befriended in Mumbai, Headley was a US businessman who ran the Mumbai branch of his friend Rana's Chicago-based firm, Immigration Law Centre.
He volunteered to fight in Kashmir, but at 40, was rejected for being too old.
"He establishes the complicity of the Pakistan Army and Inter-Services Intelligence (ISI) and also indicts Lashkar-e-Toiba (Le T) supremo Hafiz Muhammad Saeed," he says.
Pakistani courts let Saeed off in 2009 for insufficient evidence.
The drifter found his ideological moorings within an organisation determined to bleed India.
By 2002, the DEA double agent had became a triple agent.