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

Is David Coleman Headley telling the truth?

That question will continue to hover over a packed courtroom in Chicago this week as the 50-year-old Pakistani-American businessman resumes his testimony in a high-stakes terrorism trial involving the 2008 Mumbai attacks. The trial could have a profound impact on the troubled relationship between the United States and Pakistan because Headley has asserted that Pakistani intelligence officers played a key role in the attacks.

By his own admission, Headley has credibility problems.

He is a former heroin addict and drug smuggler. He has juggled allegiances to the DEA, the Lashkar-e-Taiba terrorist group, al-Qaida and Pakistani intelligence. He has maneuvered among overlapping relationships with three wives, including the mother of his four children. To save himself from the death penalty, he has pleaded guilty to doing reconnaissance for the Mumbai attacks and a plot in Denmark and is now the star witness against Tahawwur Rana, his boyhood friend and accused accomplice.

When the FBI arrested Headley in 2009, investigators were stunned by his insider’s knowledge of the Mumbai plot, which killed 166 people, six of them Americans. At the same time, however, they worried that his prodigious talent for deception could result in disaster in court, so they worked around the world to confirm as much of his account as they could.

They scoured the trove of information in his computer. They analyzed his phone, travel and credit card records. They pored over the intelligence haul from at least two months they had spent shadowing him and monitoring his communications before his arrest. They compared his story to the results of investigations in India, Pakistan, Denmark, Britain and elsewhere.

As a result, the case unfolding in Chicago consists of far more than Headley’s word.

When Headley testified last week that he met a mastermind in Karachi as Lashkar prepared to deploy a maritime attack team, the prosecution produced his hotel bill from that date in Karachi. (Investigators had previously corroborated aspects of his account of the preparations in Karachi by comparing it to the confession of the surviving gunman.) When Headley described scouting targets in Denmark, prosecutors showed the jury his surveillance video of those targets. At some points, his testimony and the supporting evidence flowed together to create an almost real-time picture of his activity.

Headley’s most eagerly awaited testimony involves Pakistan’s Inter-Services Intelligence Directorate (ISI) and centers on a shadowy figure known as Major Iqbal. Headley says Iqbal was the ISI handler who trained, directed and funded him, though he admits he does not know Iqbal's real name.

Pakistani officials have denied that the ISI played any role in the Mumbai attacks and that Iqbal was a serving intelligence officer. Some question whether Iqbal really exists.

But U.S. prosecutors are so convinced that Major Iqbal is real that last month they took the diplomatically explosive step of indicting him. They have done their best to bring him to life in the courtroom, displaying his email exchanges with Headley and Rana. It seems clear that Iqbal was running Headley in coordination with Lashkar but at the same time was directing him to collect the kind of military intelligence that interests a spy agency, not a terrorist group. Iqbal also asked Headley to look into purchasing espionage equipment in the United States, evidence shows.

The trial has featured phone evidence, including a number Iqbal obtained with a New York area code to disguise his calls from Pakistan to India. According to intercepted phone calls and retrieved emails, Headley spent months talking with associates about Iqbal and other ISI officers. His view of his Lashkar and ISI handlers soured in the spring of 2009 when, after launching him on the plot against Denmark, they shelved the operation and he began working with al-Qaida instead.

In September 2009, Headley received a call in Chicago from his brother in Pakistan saying that Major Iqbal had come to Headley’s house in Lahore looking for him, according to evidence from a wiretap. Headley responded with an obscene insult about his former handler. In other communications, Headley complained to an al-Qaida operative that Lashkar only did the bidding of the ISI and that he should have asked his ISI contacts to help him get a long-term Pakistani visa.

Iqbal was not Headley’s only point of contact with the spy agency. Headley has described meeting several other high-ranking officers. The prosecutors have unveiled his communications with a Major Sameer Ali, whom Headley describes as an ISI officer. Emails show that Ali helped the American find out that Headley’s al-Qaida handler, who also had a relationship with the ISI, had been released after a brief detention in 2009. Investigators have determined that Major Ali worked closely with Major Iqbal, though that evidence has not yet been presented in court.

Headley remains the sole source of some information produced in the courtroom, including his account of the spy training he received at a safe house in Lahore, where he says his instructors were sergeants, corporals and other non-commissioned officers working for Major Iqbal. Experts say Headley’s tradecraft as a reconnaissance operative suggests that he did, in fact, have professional training. The meticulous advance work and tactical sophistication of the Mumbai plot far exceeded the majority of operations by al-Qaida and other groups working without state support.

When Rana’s defense attorneys continue their cross-examination of Headley this week, it’s possible that they will severely damage Headley, or that he will self-destruct on the stand. But so far the defense’s approach has tended to reinforce his credibility.

In fact, Rana’s lawyers appear to accept Headley’s claim that he worked for the ISI and even see it as a factor that mitigates Rana’s guilt. They assert that Rana, who communicated with Major Iqbal but not with the Lashkar masterminds, thought he was helping Headley conduct espionage operations for the ISI when he let him use his Chicago immigration consulting firm as a cover. The defense depicts Headley as skilled manipulator who kept his childhood friend in the dark about his terrorist activity while using him as an unwitting accomplice.

The lawyers accuse Headley of lying to implicate Rana in order to save his wife as well as his own life. Headley admitted on the stand Thursday that his wife Shazia, who has not been charged, knew of the Mumbai and Denmark plots. The defense quoted a congratulatory email she sent him as the carnage in Mumbai filled television screens worldwide.

The impression Headley makes on the stand could determine whether the jury convicts Rana—and whether Americans who are following the trial believe Pakistani intelligence officers took part in a plot to kill Americans.

So far he has come off as sophisticated, tormented and intense, speaking in precise, clipped sentences with a tinge of a South Asian accent. He has veered from ruthless to sentimental, from slick to vulnerable. He justifies the killing of innocents in Mumbai as revenge for the killings of innocents in Pakistan by India in past wars. Although he dropped out of two military schools, he sees himself as an Islamic warrior and hopes that his 5-year-old son will grow up to be a commando. He recounted an anecdote in which his son, told by a soccer coach on the field to shoot, dropped into a combat stance imitating his father practicing on a target range at their house in Lahore.

But Headley’s attitude toward his murderous exploits has evolved during his testimony. At one point he said he was “pleased” in 2008 when he saw the televised news of the three-day slaughter in Mumbai. But when asked Thursday if he was still proud of his role in the attacks, he paused and said, “No.”