ProPublica's Sebastian Rotella has spent more than a year investigating suspected links between Pakistan's intelligence service and terrorist groups as well as the failure of the U.S. government to detect the growing threat posed by the Lashkar-i-Taiba militant group. He has traveled overseas and around the United States to track down secret documents and conduct exclusive interviews with counterterrorism officials and people close to David Coleman Headley, a Pakistani-American who has confessed to doing reconnaissance work in Mumbai for Lashkar. Rotella has written more than 33,000 words on the subject, which might seem a bit overwhelming if you're a newcomer to the subject. So here's an overview of the basics to get you started, plus links to stories that can help you dig even deeper.

What's the significance of the terrorism trial going on in Chicago?

The United States has indicted seven suspects in the three-day string of terrorist attacks in Mumbai in November 2008 and a follow-up plot in Denmark. The attacks killed 166 people and wounded 308. Because six of the dead were U.S. citizens, federal prosecutors and the FBI are required by law to pursue an investigation.

On May 23, a Chicago immigration consultant, Tahawwur Rana, went on trial in Chicago federal court on charges of material support of terrorism. The trial has attracted worldwide attention because, for the first time, details about the alleged terror connections of Pakistan's Inter-Services Intelligence Directorate (ISI) are being brought to light in a U.S courtroom. Several of the alleged masterminds of the Mumbai attacks have been linked to the ISI. Though the U.S. has long suspected elements of the ISI of working with terrorist groups, those concerns have been downplayed because Pakistan is a key ally in the fight against terrorism.

The Mumbai massacre is the most spectacular strike to date by Lashkar-i-Taiba, and the first Lashkar attack that has expressly targeted Westerners. Prior to the Mumbai attacks, many U.S. officials assumed Lashkar's energies were focused on India in the struggle over the territory of Kashmir.

Some worry that publicity from the trial could interfere with the U.S. government's goal of keeping Pakistan as an ally.

For the latest from the trial, keep an eye on our investigation page.

Who are the major players in the trial?

Tahawwur Rana is on trial as an accomplice in the Mumbai attack and the Denmark plot. Prosecutors allege that he allowed Headley to use his business as a cover while performing reconnaissance missions related to Mumbai. Rana has pleaded not guilty. For more on Rana, and Rotella's exclusive interview with Rana's wife, see this piece produced by our partners at PBS Frontline.

The star witness against Rana is his boyhood friend David Coleman Headley, a former DEA informant who has confessed to doing reconnaissance work for Lashkar in the years leading up to the Mumbai attacks. Headley also did reconnaissance for al Qaeda in a failed plot to behead employees of a Danish newspaper that published a cartoon of the Prophet Muhammad. After Headley's arrest in 2009, he offered to help the FBI capture other terrorists as he sought a deal to avoid the death penalty. Headley's testimony points to a close alliance between Lashkar and ISI officers. Headley, 50, has proven a great asset to the terror groups he has worked with because of his atypical profile: his American passport and Western appearance, his age and language skills, his ability to operate in the West and South Asia and his extensive contacts in Pakistan's elite and the criminal underworld. He is possibly the most colorful character to emerge at the center of a U.S. terrorism trial. You can read more about his background here, and about the credibility of his testimony here.

Two alleged masterminds named in the indictment, Sajid Mir and a man known only as Major Iqbal, are suspected of having ISI connections. Mir was picked by Lashkar-i-Taiba to organize the Mumbai massacre: he chose targets, oversaw the plotting and directed the militants carrying out the attacks by phone from Pakistan. Some anti-terrorism officials say he is a former officer in the Pakistani military or the ISI, though others doubt he was actually in the military. Rotella put together a compelling portrait of Mir and the Mumbai attacks, "The Man Behind Mumbai." It's also available as a Kindle Single.

Major Iqbal is a suspected ISI officer who worked as a liaison to Lashkar. Headley identifies him as his ISI handler, saying Iqbal worked in tandem with Mir, Headley's Lashkar handler. Iqbal trained Headley in espionage skills separately from Lashkar, directed and funded his reconnaissance and played a key role in planning the Mumbai attack, according to trial testimony.

What is Laskhar-i-Taiba?

Lashkar was founded in the 1980s and fought against Soviet incursions into Afghanistan, an effort supported by the U.S and Pakistan. Pakistan's military used the group as a strategic ally in its fight with India over Kashmir, working so closely with Lashkar that the military often assigned officers to work with the militant group. The Pakistani government officially outlawed Lashkar after a 2001 attack on India's parliament. But David Coleman Headley testified that in the years leading up to the Mumbai attacks the ISI retained a close alliance with Lashkar and its officers helped screen and train recruits from overseas at Lashkar training camps.

Lashkar is enmeshed with other terror groups in the region. Its founder, Hafiz Saeed, was a mentor to Osama bin Laden and helped him found a group that was a precursor to al Qaeda. Al Qaeda, for its part, works with the Pakistani Taliban. The Barcelona subway bombing plot of 2008 was believed to have been a joint venture by the Taliban and al Qaeda. The Mumbai investigation showed that a number of Lashkar fighters, including former Pakistani military officers, have defected to the Taliban and al Qaeda in recent years.

Lashkar has long served as an ally for al Qaeda, providing everything from safe houses for leaders to a kind of training ground for aspiring holy warriors. Rotella reported that several led al Qaeda plots against New York and London.

After 9/11, most U.S. counterterror officials dismissed Lashkar as a potential threat because it seemed focused on India. But Lashkar increasingly seems to pose a unique threat because of its para-military discipline, popularity in Pakistan, ample war chest and longtime ties to the Pakistani intelligence service.

Jean-Louis Bruguière, a French judge who investigated Mir, said "Lashkar is not just a tool of the ISI, but an ally of al Qaeda that participates in its global jihad. Today Pakistan is the heart of the terrorist threat. And it may be too late to do anything about it."

For a rundown of Lashkar's origins and a fuller account of the plot behind the Mumbai attacks, read "The Man Behind Mumbai."

How did the U.S. handle warnings about Headley?

Beginning in 2001, five people close to Headley warned U.S. officials about his dealings with terrorists, including two of Headley's three wives. They described his radicalization, his training in Lashkar camps and his apparent missions in Pakistan and India. But the officials said their allegations were too general, didn't point to a particular plot and could have been motivated by personal grudges. Despite the repeated warnings about Headley's terrorist involvement, he wasn't arrested until 2009, 11 months after the Mumbai attacks. Headley served as a DEA informant starting in the late 1990s and testimony revealed that he was still an informant when he trained in Lashkar's camps in 2002. Mysteries persist about the nature of his work as an informant, when it ended and whether it shielded him from more aggressive scrutiny by the FBI.

See this story for details on the substance of the tips, and how they were handled by U.S. officials. As a result of Rotella's stories last October, the Office of the Director of National Intelligence reviewed the U.S. government's handling of Headley. Its findings have not been made public.

What happened with the Denmark plot?

Headley testified in Chicago that the plot to attack the Danish newspaper was launched by the ISI and Lashkar, but they shelved the project after his first reconnaissance trip. The plot then shifted to al Qaeda, with kingpin Ilyas Kashmiri directing and funding Headley's reconnaissance and attempts to recruit an attack team in Europe. News reports from Pakistan this weekend indicate Kashmiri may have been killed in a U.S. missile strike, but U.S. officials have not yet confirmed that. ProPublica and PBS Frontline are working on a documentary about the Mumbai and Denmark terror plots that will air in the fall.

In this video, Sebastian Rotella walks you through the details of Headley's reconnaissance work:

Watch the full episode. See more FRONTLINE.