David Coleman Headley, 52, faces a maximum life prison term when US District Judge Harry Leinenweber sentences him for his role in a three-day rampage in which 10 gunmen from a Pakistani-based militant group fanned out across Mumbai, attacking a crowded train station, the landmark Taj Mahal Hotel and other targets. Around 160 people were killed, including children.
Prosecutors, though, are asking for a relatively lenient term of 30 to 35 years, which leaves open the possibility Headley one day could go free. Headley seemed to leap at the chance to spill secrets following his 2009 arrest and continued providing details even after the US government agreed not to seek the death penalty in exchange for his cooperation.
Prosecutors say Headley, who was born in the US to a Pakistani father and American mother, was motivated in part by his hatred of India going back to his childhood. He changed his birth name from Daood Gilani in 2006 so he could travel to and from India more easily to do reconnaissance without raising suspicions.
He never pulled a trigger in the attack that's been called India's 9/11, but his contribution to the Pakistani-based militant group, Lashkar-e-Taiba, made the assault more deadly. He conducted meticulous scouting missions - videotaping and mapping targets - so the attackers who had never been to Mumbai adeptly found their way around.
"What he did was unfathomable," said James Kreindler, an attorney for relatives of American victims. "Imagine what is going through a person's mind who is videotaping these places knowing what will happen there later."
One survivor of the deadly attack said a lighter sentence would be "an appalling dishonour" to those killed.
"I feel that for the magnitude of the killings that took place, David Headley has lost his right to live as a free man," said Kia Scherr, whose husband and daughter died in the attack. "This would be a moral outrage that is inexcusable."
Prosecutors also have praised Headley for testifying against Tahawwur Rana, a Chicago businessman convicted of providing aid to Lashkar and backing a failed plot to attack a Danish newspaper for publishing depictions of the Prophet Muhammad. Rana, sentenced last week to 14 years in prison, claimed his friend Headley duped him.
Testifying at Rana's trial in 2011, Headley spoke in a monotone voice, seemingly detached, even as he described one proposal for the never-carried-out Danish plot to behead newspaper staff and throw their heads onto a street.
In video excerpts of his interviews with the FBI after his arrest, Headley appears flippant, cool and calculating. As he revealed Rana's name, he told an investigator in an upbeat voice, "That probably is going to be good a plus for me. Also for you."
In big cases where suspects cooperate, prosecutors often ask for leniency. It's both a reward and a message to future suspects that they, too, could get a break if they spill their secrets. Still, for a reviled figure like Headley to get a sentence less than sentences routinely meted out to convicted drug traffickers or child pornographers could prompt criticism.
Prosecutors seemed to anticipate that in their filing, acknowledging that, "Determining the appropriate sentence for David Headley requires consideration of uniquely aggravating and uniquely mitigating factors."
Prosecutors have recounted only in broad terms how Headley has shed light on the leadership, structure and possible targets of Lashkar-e-Toiba, which was believed to have ties to the Pakistani intelligence agency known as ISI. Headley has said his ISI contact was a "Major Iqbal," who was named in the indictment that charged Headley.
Seth Jones, a RAND Corp political scientist, agrees Headley must have provided useful insight for American intelligence, especially about how Pakistani intelligence agents allegedly reach out to people like Headley.
"From my perspective, this was pretty detailed information about one ISI contact (Headley) with one handler, Iqbal," said Jones. But he added Pakistani intelligence would have been careful not to reveal too much to Headley, saying, "They didn't trust him either."
For his cooperation and guilty plea to 12 counts, Headley secured both a promise that he would not face the death penalty and would not be extradited to India. Late last year, India secretly hanged the lone gunman who survived the Mumbai attack, Mohammed Ajmal Kasab.
The 12 counts Headley pleaded guilty to included conspiracy to commit murder in India and aiding and abetting in the murder of six Americans, who included Americans Alan Scherr and his 13-year-old daughter, Naomi.
Scherr and her family were in India for a two-week spiritual retreat and were staying at the Oberoi Trident Hotel, one of the sites that came under assault.
After the attack, Scherr helped start an organization called the One Life Alliance, which seeks to work against terrorism by promoting understanding and respect for the sacredness of life.
"This is how I am surviving this event, which erased life as I knew it," she wrote in an email from Mumbai, where she continues to travel to for charity work.
Another survivor, Andreina Varagona, described in a presentencing filing dining with the Scherrs at the hotel restaurant when gunmen burst in. Bullets tore apart the room as they dove under a table, the girl screaming.
"I suddenly felt the warm spray of blood on my face and in my hair. ... Naomi's screams had stopped too, and I saw her lying lifeless beside (her father)," she recounted. "They'd both been shot dead."