David Coleman Headley, who helped plot the devastating 2008 Mumbai attacks before agreeing to become an informer, was sentenced by a US judge on Thursday to 35 years in prison.
Headley (52) struck a deal to avoid the death penalty by pleading guilty to scoping out Mumbai on behalf of Pakistani militants and to a second plot to attack a Danish newspaper over cartoons mocking the Prophet Mohammed.
"The sentence I impose, I'm hopeful it will keep Mr Headley under lock and key for the rest of his natural life," Judge Harry Leinenweber said.
He added that it would have been much easier to impose the death penalty, saying: "That's what you deserve". He opted, however, for a 35-year sentence after a motion by the government, saying it was "not a light sentence".
Heavily-armed militants rampaged through Mumbai in November 2008, killing 166 people and wounding hundreds more over nearly three days of carnage in a prolonged assault on the Indian financial capital.
But US attorney Patrick Fitzgerald had urged leniency, telling the judge that Headley's decision to become an informant "saved lives".
In a plot that reads like a spy thriller, Headley spent two years casing Mumbai, even taking boat tours around the city's harbor to find landing sites for the attackers and befriending Bollywood stars as part of his cover.
Prosecutors described it as a supporting but "essential" role.
Victims of the 2008 attack had called for a harsher sentence as they recounted scenes of horror during the three-day assault.
Linda Ragsdale (53) broke into tears as she described gunmen bursting into the Oberin hotel restaurant and slaughtering her friends, leaving her with a one-metre-long bullet scar.
"I know the sickeningly sweet smell of blood and gunpowder. I know the sound of life leaving a 13-year-old child. These are things I never needed to know," she told the judge ahead of sentencing.
She said it would be an "appalling dishonour" and a "moral outrage" if Headley received only 35 years in prison for his crimes.
Addressing Headley, Ragsdale said: "I do not wish you death, but total silence and isolation for you to commune with your higher power."
The Washington-born son of a former Pakistani diplomat and American woman, Headley's Western appearance and US passport helped him slip under the radar for much of the seven years he spent working with militant groups.
And while he quickly turned informant to save his own skin, prosecutors said Headley was committed to the cause of terrorism.
He was so eager to attack Denmark's Jyllands-Posten newspaper over its publication of cartoons mocking the Prophet Mohammed that he began working seriously on that plot two months before the Mumbai attack.
He also had Bollywood and one of India's most sacred Hindu temples in his sights as he began plotting a second India attack during a March 2009 surveillance trip.
India objected after US prosecutors took the death penalty off the table and agreed not to extradite Headley in exchange for his cooperation after his October 2009 arrest in Chicago as he was set to board a flight to Pakistan.
US prosecutors have kept most of the details of Headley's cooperation under seal but say the information he began to provide "immediately" after his arrest proved too valuable to pass up.
"We had to because it's too important that we do everything we can to save lives," Fitzgerald told reporters after the 2011 conviction of Headley's childhood friend and co-conspirator Tahawwur Hussain Rana.
Rana (52) was sentenced to 14 years in prison last week for letting Headley use his Chicago-based immigration firm as a cover while working on the Denmark plot for Lashkar-e-Taiba (LeT), the group India blames for the Mumbai attacks.
Fitzgerald has said that Headley provided details about dozens of potential targets in India and Denmark that were under surveillance.
Headley — who changed his name from Daood Gilani so he could hide his Pakistani heritage — joined LeT in 2002, attending terrorist training camps five times over the next three years.