his cooperation" as reason for seeking a lighter sentence.
Headley will be sentenced by that court on Thursday. He was never tried as he pleaded guilty to all 12 charges in exchange for immunity from extradition and death penalty.
A Pakistani American, Headley visited Mumbai many times between 2006 and 2008 to scout the targets for his Lashkar and ISI handlers. He also picked the landing area used by the attackers to sail in undetected.
Tahawwur Rana, a Canadian who gave Headley the cover of his immigration business for the scouting missions, was sentenced to 14 years by the same court last week.
India hanged Ajmal Kasab, the only other Mumbai carnage perpetrator or planner arrested outside Pakistan, last November, four years after the attacks.
He was one of the 10 men sent by Lashkar. The rest were killed.
Headley's offenses, as pointed out to him by US law enforcement officers during his interrogation, said the pre-sentencing paper, could have gotten him the death penalty.
The prosecution, however, is not seeking death for Headley and not even life imprisonment. It is asking for a sentence "at a point below the life imprisonment".
"The government submits that imposing a sentence of 30 to 35 years' imprisonment strikes a fair and just balance between the despicable nature of his crimes and the significant value of his cooperation," said the position paper filed by Gary S Shapiro, acting US attorney in the Chicago court on Tuesday.
Headley, a one-time agent of the US Drug Enforcement Administration, began cooperating with authorities immediately after his arrest at the Chicago airport in November 2009.
The position paper here points that at this stage Headley was cooperating unconditionally, without any terms or a deal promising him immunity.
"After being specifically advised that, based on the information that he was providing, he likely would face charges that carried the death penalty in the United States, Headley continued to speak with investigators for two weeks before his arrest became public," said the position paper.
At the direction of the authorities he also contacted several associates during this period, helping investigators identify many members of Ilyas Kashmiri's outfit.
Headley was planning the Danish newspaper attack with Kashmiri, who headed a terrorist outfit called the 313 Brigade and often worked with al Qaeda.
He was killed in a US Drone attack in 2011.
The prosecution said Headley provided extensive details about Lashkar-i-Tayyeba: its leadership, training methods, funding, recruitment, and the way it planned attacks.
The former agent also helped the US bring charges against at least seven other people, including Rana, said the position paper. Headley deposed extensively during Rana's trial.
The other six charged with Headley's help were: Sajid Mir, his handler and key Mumbai carnage plotter; Abu Qahafa, who trained the 10 attackers; Mazhar Iqbal, one of their controllers; Major Iqbal, an ISI officer who helped in the planning and funding of the attack; Kashmiri; and Abdur Rehman Hashim Syed, a Kashmiri associate.
Other details of Headley's cooperation were not made public, and were given to the court under seal as a "Classified Supplement" to the position paper.
The paper also said Headley has since cooperated with Indian law enforcement officers who spoke to him over seven days -- in 2010, many months after his arrest -- and "found the information to be useful".
India wanted to try him, but Headley was now protected from extradition.