In a submission before a Chicago court, the US government said Headley provided video of and intelligence about the locations that later were targeted during the Mumbai terrorist attacks.
He also assisted in plotting out a portion of nautical route that the attackers would take, and recommended the landing point that they later used to enter Mumbai without being detected, it said.
"Headley played a supporting role to the attacks, but an essential one that unquestionably contributed to the mass murder that took place," US government informed the Chicago Court through Attorney Gary S Shapiro.
According to Shapiro, from 2002 to 2005, Headley attended five separate Lashkar training camps where he was indoctrinated on the merits of waging jihad and trained in combat skills.
When Lashkar leaders decided that Headley would better serve them by conducting advance surveillance, he changed his name and enlisted the assistance of his friend, Rana, to travel to and stay in Mumbai without detection starting in 2006 and continuing to 2008, Shapiro said.
Shapiro told the court that before the Mumbai attacks took place, Headley was instructed by his Lashkar handler to travel to Denmark to perform surveillance for yet another attack.
After seeing what took place in Mumbai in Nov 2008, Headley traveled to Denmark in Jan 2009, he said.
There, he gained entry to two separate facilities for the newspaper, using the same cover story provided to him by Rana that he used in Mumbai, Shapiro said, adding that in Feb and May 2009, he met with Ilyas Kashmiri, who encouraged Headley to break away from Lashkar and plan the attacks in Denmark with his group.
"Kashmiri directed Headley to meet with associates in the United Kingdom, and inform them that the plan was to take over the newspaper building and fight to the death with responding Danish forces. Specifically, Kashmiri told Headley that he wanted the attackers to behead hostages and throw the heads on the street in order to heighten the response from Danish authorities," it said.
According to Shapiro, Headley traveled to the UK to meet with those associates in July 2009, and thereafter visited Copenhagen a second time, where he took more video of the intended target.
In early Oct 2009, Headley was arrested at O'Hare Airport, where he was set to travel to Pakistan to again meet with Kashmiri as well as Sajid Mir.
"In sum, Headley worked to advance the violent goals of terrorist organizations and terrorist attacks for approximately seven years," Shapiro said.