The Pakistani-American who spent months casing out Mumbai ahead of the 2008 attacks told a Chicago jury that Pakistan's spy service supports the extremist group he worked with on the deadly siege.
David Coleman Headley testified that Pakistan's powerful Inter-Services Intelligence (ISI) provided financial, military and moral support to banned militant group Lashkar-e-Taiba (LeT).
"I assumed these groups operated under the same umbrella -- they coordinated with each other," Headley told the jury as he described his militant path and the bloody 60-hour siege of India's largest city that left 166 people dead.
Headley described his relationship with an ISI handler identified as "Major Iqbal" who gave him $25,000 to help cover the cost of his surveillance work.
"I told him what kind of (terrorist) training I did and he mentioned he was part of Lashkar," Headley said.
Headley testified that after he told Iqbal of a 2003 plot he was working on, Iqbal asked him "to do something that was more important, which was intelligence work for the ISI."
Headley formally admitted to 12 terror charges in March 2010 after prosecutors agreed not to seek the death penalty or to allow him to be extradited to either India, Pakistan or Denmark to face related charges.
He was testifying at the trial of an old friend from military school in Pakistan -- Chicago businessman Tahawwur Hussain Rana -- who stands accused of providing Headley with a cover and acting as a messenger.
The proceedings are likely to add fuel to a diplomatic crisis over suspicions of official Pakistani complicity with terrorism after US commandos killed Al-Qaeda leader Osama bin Laden on May 2 in a northwestern garrison city deep in Pakistan.
While US prosecutors named ISI officers as co-conspirators in the Mumbai attacks, it remains unclear whether they were acting independently or with the approval and aid of senior officials.
The Washington-born son of a former Pakistani diplomat and an American woman, Headley admitted to spending months scoping out sites for the Mumbai siege and plotting to kill a Danish cartoonist.
In a plot that reads like a movie thriller, Headley spent two years scouting Mumbai, even taking boat tours around the city's harbor to identify landing sites for the attackers and befriending Bollywood stars as part of his cover.
The attacks stalled a fragile four-year peace process between the two South Asian neighbors and nuclear-armed rivals which was only resumed in February.
Rana -- who holds Canadian citizenship -- insisted after his 2009 arrest that he was a pacifist "duped" into letting his old friend use his immigration services company as a cover.
In opening statements Monday, his lawyers tried to undermine Headley's credibility and insisted that Rana thought he was helping the ISI, not extremists.
But prosecutors said Rana knew exactly what he was doing, and told jurors they would hear secret recordings in which Rana praised the Mumbai attackers.
"The defendant knew what Headley was doing and he helped him," said US Attorney Sarah Streicker.
None of the other co-conspirators is in US custody and a spokesman for the US Attorney's Office declined to say whether the US government would be seeking their extradition or even knows their whereabouts.
Headley -- who changed his name from Daood Gilani so he could hide his Pakistani heritage -- joined LeT in 2002, attending training camps five times over the next three years.
He began working with an Al-Qaeda-linked group in Pakistan called Harakat-ul-Jihad al-Islami on the Danish plot after LeT became preoccupied with the final planning for the Mumbai attack.
Headley was arrested at Chicago's O'Hare airport on his way to deliver 13 surveillance videos he obtained after pretending to be interested in buying ads in Jyllands-Posten, Denmark's highest circulation daily.
The newspaper triggered a furor in the Muslim world by publishing 12 cartoons of Prophet Mohammed in 2005.
Rana's trial is expected to last about four weeks.