This report is part of a ProPublica and PBS FRONTLINE investigation.

A version of this article appeared in the Washington Post.

A confessed Pakistani-American terrorist took the stand in a Chicago courtroom on Monday and described a close alliance between Pakistan's intelligence service and the Lashkar-i-Taiba terrorist group, alleging that Pakistani officers recruited him and played a central role in planning the 2008 Mumbai attacks.

David Coleman Headley's long-awaited testimony at the start of a trial with international repercussions resolved one question at the outset: U.S. federal prosecutors did not hesitate to connect Pakistan's Inter-Services Intelligence Directorate (ISI) to the attacks that killed 166 people, including six Americans.

Headley has pleaded guilty to doing reconnaissance in Mumbai and is the star government witness against his accused accomplice, Tahawwur Rana. Headley testified that Lashkar "operated under the umbrella of the ISI" even after the group was banned in Pakistan in 2001.

The ISI and Lashkar "coordinated with each other," Headley testified. "And ISI provided assistance to Lashkar: financial, military and moral support."

After he trained three years with Lashkar, Headley said a "Major Ali" of the ISI recruited him when he was briefly detained near the Afghan border in 2006. Major Ali referred him to an officer known as Major Iqbal, who became Headley's handler and worked separately but in coordination with Lashkar chiefs, directing Headley's reconnaissance in India and providing $25,000 to fund his mission.

Before reporting to Lashkar, Headley always reported first to Iqbal, who participated in key aspects of the Mumbai plot such as target selection, the route for an amphibious attack and a proposed safe house for Lashkar gunmen, Headley testified.

"This was being coordinated," Headley said. "But the instructions emanated from Major Iqbal."

Pakistani officials have repeatedly denied any ISI role in the Mumbai attacks. But Monday's testimony is likely to intensify allegations that Pakistan plays a double game in the fight against terrorism, especially after Osama bin Laden was found hiding in a military town near the Pakistani capital.

After more than a year of official silence about the sensitive case, the prosecution's examination of Headley quickly laid out evidence implicating the ISI in a sophisticated commando-style attack that was designed to kill Americans, Jews and other Westerners.

To corroborate Headley's allegations, prosecutors displayed evidence of his email exchanges and phone calls with Major Iqbal. Headley testified that Iqbal had acquired a phone number with a New York area code and told him to call it while in Mumbai to avoid detection "because all calls, almost, from India to Pakistan are monitored."

The case rests largely on the credibility of Headley, a former drug dealer and DEA informant. Headley has spent months helping the prosecution prepare and has cooperated enthusiastically in drug cases in the past.

But he seemed remarkably subdued Monday. The tall, broad-shouldered and balding 50-year-old took the stand wearing a blue track suit and a solemn expression. Asst. U.S. Atty. Daniel Collins asked him repeatedly to speak more loudly. The multilingual, well-traveled Headley, who speaks with a trace of a Pakistani accent, often sighed or sounded exasperated as he responded.

Parts of the account Headley gave Monday had already surfaced in an Indian interrogation report and been disclosed to ProPublica by Western and Indian investigators. But there were new details as the evidence was aired in a U.S. courtroom packed with U.S., Indian and Danish journalists.

Headley described his close relationship Lashkar's top leaders and said they alluded casually to their alliance with the ISI. At a lunch in Pakistan 2004, Headley suggested to Hafiz Saeed and Zaki Ur-Rehman Lahkvi, the group's spiritual and military chiefs, that they sue the U.S. government for having designated Lashkar a terrorist organization, according to his account.

"Zaki said we should take the ISI into confidence before making such a big move," Headley testified.

The Pakistan military also supported Lashkar at the tactical level, Headley alleged. Headley testified that a Pakistani Navy frogman provided technical advice in a meeting at Lashkar headquarters to plan the route for the maritime attack. Headley said the debate about a landing site indicated that the frogman would accompany the gunmen to Mumbai. Headley testified that the frogman warned the plotters that a proposed landing site near an Indian naval base might force him into a shootout with Indian authorities.

Rana, Headley's boyhood friend, is the only defendant on trial. The others indicted include Major Iqbal, three Lashkar chiefs and two al-Qaida figures accused of overseeing Headley's scouting for a foiled plot in Denmark. Rana is charged with material support of terrorism for allowing Headley to use his Chicago-based immigration consulting firm as a cover for his reconnaissance.

In his opening statement, Rana's lawyer argued that Headley had manipulated Rana just as he had manipulated multiple wives, extremist groups and government agencies. Charles Swift also raised the possibility that Headley may have continued working for the U.S. government well after he started training with Lashkar. He accused Headley of changing his story to betray Rana and avoid the death penalty.

"Will David Headley make a fool out of us?" Swift asked the jury. "I'm confident he will not. ... The people who did this should be held accountable. The tragedy is that we made a deal with them."