A controversial deal cut with a key figure in the deadly 2008 Mumbai attacks saved lives and provided key intelligence about Pakistani terrorist groups, a top US prosecutor said on Thursday.
The United States has come under fire in India for a plea deal reached with David Coleman Headley, who used his western appearance and US passport as a cover while carrying out surveillance ahead of the 2008 siege in which 166 people were killed.
But US Attorney Patrick Fitzgerald, who cut the deal with Headley after his October 2009 arrest at a Chicago airport, said it was well worth it to take the death penalty off the table and agree not to allow him to be extradited to either India, Pakistan or Denmark to face related charges.
"We had to because it's too important that we do everything we can to save lives," Fitzgerald told reporters.
"We would have been crazy if we would have said 'you know what, it's all about Mr Headley and we want to put him in jail' and sat around and let attacks happen."
Headley provided law enforcement with valuable intelligence stemming his work with two Pakistani terrorist groups in order to save his own skin, Fitzgerald said.
That included details on 34 potential targets in India and Denmark that under surveillance by the groups and an "insider's view" of what Ilyas Kashmiri "a very, very influential terrorist leader linked to al Qaeda" and others were planning and how they operated, Fitzgerald said.
Fitzgerald was speaking after a jury convicted Chicago businessman Tahawwur Hussain Rana on two terrorism charges based in part on Headley's cooperation with prosecutors.
The trial has been being closely watched as it touched on alleged Pakistani military intelligence collusion with terrorism -- a hugely sensitive issue after Osama bin Laden's killing sparked similar charges.
Pakistan's powerful Inter-Services Intelligence agency has long been suspected of involvement in the Mumbai attacks, and three ISI agents were named as co-conspirators by US prosecutors.
Only Rana and Headley -- who both lived in Chicago -- have been arrested and prosecutors list Kashmiri and five other conspirators as fugitives believed to be in Pakistan.
Headley testified that he believed the ISI's involvement in the Mumbai plot was limited to a handful of rogue agents, but that he believed the ISI worked closely with the banned Pakistan militant group Lashkar-e-Taiba.
He also described how he began working with Kashmiri's Pakistan-based Harakat-ul Jihad al-Islami on a plot to attack a Danish newspaper that published controversial cartoons of the Prophet Mohammed after LeT got distracted with the Mumbai plot.
Fitzgerald declined to comment on whether Headley provided details that reportedly helped US forces locate Kashmiri and kill him with a drone attack in Pakistan.
"I don't know whether there was a drone, what happened with a drone or who's alive or dead," Fitzgerald said, adding that the US government's investigation into the Mumbai attacks and other plots revealed by Headley "is not done."
"We believe a measure of justice was taken today, but we're hoping there will be other people brought to justice here or overseas," Fitzgerald said after a jury convicted Rana on two of three charges.
Fitzgerald noted that Headley is a "long way away" from being sentenced and declined to comment on how many years may be shaved off his sentence due to his cooperation with authorities.
"We need information. The American government needs it, the Pakistani government needs it, the Danish government needs it," he said.
"I have no doubt in my mind that the information we obtained as a result of that process is something that was important to obtain and very valuable and we are very comfortable with the deal we made."