CHICAGO (AFP) - US prosecutors on Tuesday urged a judge to be lenient this week when he sentences an American who admitted to scouting out Mumbai ahead of the deadly 2008 siege because of his "significant" cooperation.David Coleman Headley, 52, pleaded guilty in 2010 to 12 charges related to the carnage in Mumbai and a second plot to attack a Danish newspaper that sparked outrage over its publication of cartoons of the Prophet Mohammed. He faces up to life in prison with no chance of parole at Thursday's sentencing hearing in a federal court in Chicago, but prosecutors advocated a slightly more lenient jail term of 30 to 35 years in prison.Imposing such a sentence "strikes a fair and just balance between the despicable nature of his crimes and the significant value of his cooperation," prosecutors said.Heavily-armed militants ran rampage through Mumbai in November 2008, killing 166 people and wounding hundreds more over nearly three days in a prolonged assault on the Indian financial capital.In a plot that reads like a spy thriller, Headley spent two years casing out 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.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 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," US Attorney Patrick 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 did say that Headley had 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.