New York: LeT terrorist David Coleman Headley's explosive evidence about the "close alliance" between the terror group and Pakistan's intelligence unit ISI had a "really damaging" effect on the US-Pakistani relationship, an investigative journalist has said.

Headley's role in the Mumbai attacks is the subject of a new Frontline documentary 'A Perfect Terrorist' by ProPublica reporter Sebastian Rotella said.

It chronicles Headley's journey from the US to Mumbai and reveals what American and Pakistani intelligence officials knew about him before and after his mission.

Rotella says Headley gave specific evidence at co-conspirator Tahawwwur Rana's trial in Chicago about the close alliance between the ISI and the Lashkar terrorist group.

"[He described] how the training works, how the funding works, how the coordinated decision-making works, and how they set out to do this attack together," says Rotella.

"He talks about names, places, and communications. He's a gold mine for showing how this double game in Pakistan is really played."

"It's had a really damaging [effect] on the US-Pakistani relationship, and I think really helped change the way a lot of people in the US government see their relationship with US security forces," he says, "partially because three years have gone by, and except for a couple of token arrests, the masterminds [behind Mumbai] are free."

During the trial, Headley described his meetings with both ISI and Lashkar officials before the Mumbai operation.

 He also described meeting a Pakistani military official at Lashkar headquarters who gave Lashkar advice on how to carry out a maritime attack.

"Because of his evidence, the US attorney's office in Chicago indicted Major Iqbal, [a Pakistani intelligence official], which is the first time you have a serving Pakistani intelligence officer charged in the murder of Americans," says Rotella.

Rotella said in the course of his reporting he spoke to several people about how Headley was able to train with Islamic militants and meet with al-Qaida operatives without ever drawing notice from American authorities.

"Very serious US forces tell me that the reality is, in the real world, it's not as easy as you think beforehand to detect a terrorist," he says.

"But the fact is, the guy gets away with it, and he puts together this incredible blueprint that's carried out in Mumbai.

And most people think that if it hadn’t been for the scouting he did, the Mumbai attacks could not have been pulled off the way they were because they were absolutely reliant on the surveillance and the reconnaissance and the planning he helped do."

During this time, Headley made several trips to Mumbai to plan which sites the terrorists should attack.

 "He's able to spend time in these luxury hotels and these places Westerners go and do reconnaissance that just wouldn't be possible for 95 per cent of the otherwise very capable operatives that Lashkar has," says Rotella.

 "He's unique in this sense. [In the Taj] the gunmen know the place inside and out, even though they've never been anywhere near Mumbai, let alone a luxury hotel.

That was all thanks to this preliminary work that Headley had done."

Headley was arrested in Chicago in 2009 and charged with planning terrorist attacks in India and in Denmark, where he was involved in a plot to attack a Danish newspaper that had published cartoons of the Prophet Muhammad.

A year later, Headley pleaded guilty in a deal that let him avoid the death penalty and extradition to India, Pakistan and Denmark.

In the plea bargain, he was obligated to testify against his friend from a Pakistan military school Rana, who was also charged with helping to plot the attacks in Mumbai.