One survivor of the deadly attack said a lighter sentence would be "an appalling dishonor" 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-Taiba, 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.