David Headley, 52, pleaded guilty to several charges that accused him of conducting scouting missions ahead of the three-day attack, which has been called India's 9/11. The charges carry a maximum sentence of life in prison.
But in documents filed in U.S. District Court in Chicago, federal prosecutors asked a judge to sentence Headley to between 30 and 35 years in prison. They said he should be credited for "the significant value provided by his immediate and extensive cooperation" following his 2009 arrest.
Headley provided "insight into the personnel, structure, methods, abilities and plans" of the Pakistani group that carried out the attack, Lashkar-e-Taiba, according to the filing. He also testified against Tahawwur Rana, a Chicago businessman eventually convicted of providing aid to the group and backing a failed plot to attack a Danish newspaper after it published depictions of the Prophet Muhammad.
"Obviously he is being rewarded for cooperating and testifying against Rana, and that's a good thing," said attorney James Kreindler, who represents the families of some American victims of the Mumbai attack. "Given his age, 35 years may be pretty close to a life sentence."
Headley's attorney, John Theis, declined comment.
The government's court filing outlines details of the attack on Nov. 26, 2008, noting the "staggering" death toll included many children and acknowledging that "the far-reaching and devastating impact of those attacks may never subside."
Prosecutors recount how Headley's meticulous reconnaissance, including his suggestion about precisely where the terrorists could make an amphibious landing in Mumbai, made the assault all the more deadly.
But the court documents also highlight how Headley agreed to talk almost immediately after his arrest at O'Hare International Airport in Chicago, even after he was told his answers could lead to his conviction for crimes that call for the death penalty.
His sentencing hearing is scheduled for Thursday.
Under his 2010 deal with prosecutors, Headley agreed to plead guilty and cooperate in exchange for a promise that he would not face the death penalty or be extradited to India.
Tuesday's filing offers few specifics but says Headley helped shed light on Lashkar's structure, leadership, other planned attacks and potential targets. It also said he provided valuable information about Ilyas Kashmiri, al-Qaida's former military operations chief in Pakistan who was reportedly killed in a drone strike in 2011.
Headley also submitted to interviews by Indian authorities for seven days, the filing states.
"Headley answered their questions without any restriction, and the government understands that the Indian government found the information to be useful," the court filing states.
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