Karen Sorenson, a Danish journalist and author of 'A mind of a terrorist', a book on Headley, was speaking at Times LitFest here.
Security expert Maroof Raza moderated the interaction.
Prior to Headley's arrest, US agencies had not thought that a white-skinned person could be involved in terror activities, unlike those from the Middle-East and Asia, but Headley belied this thinking, Sorenson said.
Sorenson said how a US national like Headley, born in Washington and having worked for US security agencies, ended up in LeT and played double agent was a mystery. It was the British who found about his terror involvement, he said.
More From This Section
The US needed to answer these questions (about failure to zero in on Headley earlier), Sorenson said.
The soft-spoken Headley, who wore Armani suits and claimed to be businessman, was a "perfect terrorist", the journalist said.
The purpose of Mumbai attack was to destabilise India and create a fear among its citizens. Headley's arrest also proved Pakistan's ISI and LeT were behind this attack, he said.
Headley, lodged in prison in US, will die in prison, he said.
Speaking during the discussion, Raza said operations like 26/11 cannot be carried out without help from within.