The chief prosecutor in a landmark terrorism trial that ended last week in Chicago says a plea bargain with a confessed American terrorist was justified because of his value as a source of intelligence and as a key witness in any future prosecutions.

Jurors last Thursday convicted Tahawwur Rana, a Chicago businessman, after a trial that revealed unprecedented details about the alliance between Pakistani militant groups and that country's intelligence service. In addition to investigative work by the FBI in the United States, Pakistan, India and Denmark, the case centered on five days of testimony of David Coleman Headley, who confessed to doing reconnaissance for the 2008 Mumbai attacks and a failed plot in Denmark.

Jurors convicted Rana on two of three counts of support of terrorism for letting Headley, a childhood friend, use his immigration consulting business as a cover for his plotting overseas. Headley described the Mumbai attacks as a joint operation directed by Pakistan's Inter-Services Intelligence Directorate (ISI) and the Lashkar-i-Taiba militant group. He testified as part of a plea agreement that enabled him to escape the death penalty for his role in the killings of 166 people, including six Americans, in Mumbai.

Defense attorneys argued that using Headley, a former drug dealer and DEA informant, to go after Rana was comparable to using a whale to catch a minnow.

But in a telephone interview Friday, U.S. Attorney Patrick Fitzgerald of the Northern District of Illinois said the information that Headley provided about the inner workings of terrorist groups and the ISI was unprecedented in its scope and detail.

Headley will testify in any future prosecutions of fugitive masterminds such as al-Qaida chief Ilyas Kashmiri and Lashkar's Sajir Mir, who is charged with a lead role in the Mumbai plot, Fitzgerald said. Fitzgerald declined to discuss details of the case such as the politically sensitive decision to indict a suspected ISI officer who served as Headley's handler and is known only as Major Iqbal.

"In addition to Rana, what we got from Headley was a lot of intelligence," Fitzgerald said. "There is no doubt in my mind that we would have been derelict in our duty if we didn't go after a deal with someone who had sat down with Kashmiri, with Sajid Mir, with Major Iqbal, someone who knew so much about these groups and these plots. He gave us 34 more targets in India. It was a no-brainer to me."

The trial was a duel of big guns. Fitzgerald, one of the government's toughest and best-known prosecutors, is a veteran of major terrorism cases dating back to the 1990s. He has been mentioned as a possible candidate for FBI director.

Defense attorney Charles Swift, meanwhile, is a former Navy lieutenant commander and military defense counsel who successfully challenged the legality of military commissions for Guantanamo inmates before the Supreme Court. After leaving the Navy and going into private practice, he has defended high-profile terror defendants.

In a telephone interview Monday, Swift insisted that Headley falsely implicated Rana and manipulated the government as he had when he was a DEA informant. Swift also said it is doubtful that the top suspects in Pakistan will ever be brought to trial. The FBI has developed a lot of information about the identities and whereabouts of Major Iqbal and other suspected masterminds, but Pakistan has resisted pressure from Washington to go after them, U.S. officials admit.

"The prosecutors got nothing from their deal with Headley other than his allegations against Rana," Swift said. "From my review of the evidence, they got all the significant intelligence from him before they made the deal."

The jury apparently accepted the defense argument that Headley kept Rana in the dark about the Mumbai plot by telling Rana he was doing espionage work for the ISI in India. The jury acquitted Rana of involvement in the Mumbai attacks but convicted him of supporting the failed Denmark plot and of supporting Lashkar.

Swift said the mixed verdict was not surprising.

"From the beginning we feared the verdict could be a compromise," Swift said. "That's why we asked to separate the Mumbai charges from the Denmark charges. There was more evidence on the Denmark plot. He was found completely not guilty on Mumbai."

Nonetheless, Fitzgerald said the verdict shows Rana was aware from the start that he was involved in terrorist activity. He noted that Rana admitted after his arrest in 2009 that he knew Headley had been working with Lashkar for five or six years.

"The verdicts were hardly as inconsistent as some have made them out to be," Fitzgerald said. "The charge on Mumbai required that he know enough about the plot at the time he agreed to provide support to be found guilty of specifically agreeing to aid an attack with guns and bombs. The question was when did he know the full scope of the plot. ... The jury convicted him of the substantive offense of providing material support to the terrorist group that carried out the attack but did not convict him of agreeing specifically in advance to support that attack."

The jury rejected the idea that Rana remained a dupe once the carnage in India had happened. Email and wiretap evidence showed that Rana was a willing and knowing participant in Headley's reconnaissance for an attack on a newspaper in Denmark that has become an internationally known target of terrorists after publishing caricatures of the Prophet Mohammad in 2005, Fitzgerald said.

"The jury could give Rana the benefit of a reasonable doubt as to how much he knew about the Mumbai attacks," Fitzgerald said. "But Rana played a more direct role in Denmark. ... And there was more corroborating evidence beyond Headley, whose credibility was challenged by the defense. Jurors naturally look for intrinsic corroboration. They want to see something in black and white."