In testimony that prosecutors said offered a "rare look" inside a major terrorist plot, David C. Headley said he had trained with the Islamist militant group Lashkar-e-Taiba between 2002 and 2005 in preparation for scouting locations to attack in India. In 2006, Mr. Headley said, he met a member of Pakistan's Inter-Services Intelligence Agency who offered to provide financial support for that surveillance.
In testimony so painstaking that the judge and some jurors seemed to nod off at the mundane details of a plot that left 163 people dead, Mr. Headley described how he changed his name and used his American passport to portray himself alternately as a tourist or a businessman, concealing his Muslim faith and his Pakistani roots so he could travel easily across borders. He said he provided hours of video of potential targets in Mumbai to his handlers in both ISI and Lashkar.
"I understood these groups operated under the umbrella of the ISI," he said, referring to Lashkar. "They coordinated with each other."
In her opening statements, the federal prosecutor, Sarah Streicker, echoed that comment, saying that both Lashkar and Mr. Headley's ISI contacts wanted him to "conduct surveillance on targets in India."
Mr. Headley's testimony comes at a time of deepening concerns about Pakistan's links to Islamic militants, less than a month after the United States discovered Osama bin Laden hiding out in a prominent Pakistani garrison town. Ms. Streicker did not make clear on Monday whether the government believed the Mumbai attack was plotted at the highest levels of ISI, or had the support of only a handful of rogue agents.
Still, by presenting Mr. Headley as its lead witness, the government was at least tacitly supporting his story and asking jurors to convict a defendant based largely on his testimony.
The defendant is a Chicago businessman, Tahawwur Rana, who is accused of providing support for the Mumbai attacks. In court on Monday, Mr. Rana appeared in a drab olive sport coat and smiled dolefully at members of his family seated in the front row, as if trying to console them.
Prosecutors say Mr. Rana, who runs an immigration service here, allowed Mr. Headley to portray himself as an executive of the company and open an office in Mumbai as a cover for his terrorist activities.
Defence attorneys said that Mr. Rana had no idea Mr. Headley, a friend with whom he attended an elite Pakistani military academy, was plotting with terrorists and that he had been duped. One of Mr. Rana's lawyers is Charles Swift, who successfully represented Bin Laden's driver in the landmark Supreme Court case that struck down the military commissions President George W. Bush had established to try Guantánamo Bay detainees.
In his opening statement on Monday, Mr. Swift talked more about Mr. Headley, who pleaded guilty in an attempt to avoid the death penalty, than he did his own client, describing Mr. Headley's history of deceit, including multiple marriages and working as an informer for the Drug Enforcement Administration while he was trafficking heroin into the country from Pakistan.
Mr. Swift said Mr. Headley was working as a D.E.A. informer when he started training with Lashkar, balancing his work for the militants, ISI and the D.E.A. the same way he balanced three wives.
"David Headley was living multiple lives," Mr. Swift said. "And he was very good at it."
Wearing a rumpled sweat suit and with his gray hair shaved close to his head, Mr. Headley appeared far from swashbuckling. He spoke so softly that the judge asked several times for him to keep his voice up. His answers often seemed more coached than spontaneous. And he became testy with prosecutors in a way that sometimes made it hard tell which side he was on.
Still, the case is likely to hinge on whether the jury believes his account.
On Tuesday, the defence is expected to begin its effort to poke holes in his credibility.
Calling Mr. Headley a "master manipulator," Mr. Swift told the jury that this trial is not the first time the witness has pleaded guilty in order to avoid severe sentences. Court records show he previously pleaded guilty and testified against friends in two drug convictions more than a decade ago.
"I am convinced," Mr. Swift told the jury, "that the lies and manipulations stop here."