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On a November night two years ago, a young American rabbi and his pregnant wife finished dinner at their home in the mega-city of Mumbai.

Gavriel and Rivka Holtzberg had come to India on a religious mission. They had established India's first outpost of Chabad Lubavitch, the Orthodox Jewish organization, in a six-story tower overlooking a shantytown. The Chabad House offered a synagogue, a cyber-café, two floors of guest rooms, India's biggest Hebrew library and a dining room that could seat 50 for festive meals. The Holtzbergs' guests that evening were two American rabbis, an Israeli grandmother and a Mexican tourist.

Hundreds of miles away in Pakistan, a youthful terrorist chief named Sajid Mir was preparing a different sort of religious mission. With the support of Pakistan's intelligence service, Mir had spent two years using a Pakistani-American businessman named David Coleman Headley to conduct meticulous reconnaissance on Mumbai, according to investigators and court documents. He had selected iconic targets and the Chabad House, a seemingly obscure choice, but one that ensured that Jews and Americans would be casualties.

On Nov. 26, 2008, Mir sat among half-a-dozen militant chiefs in a safe house in Karachi tracking an attack team as its dinghy approached the Mumbai waterfront. The Lashkar-i-Taiba terrorist group had made Mir the project manager of its biggest strike ever, the crowning achievement of his career as a holy warrior.

The 10 gunmen went ashore and split into five teams. His voice crisp and steady, Mir directed the slaughter by phone from his command post, relaying detailed instructions to his fighters. About 10:25 p.m., gunmen stormed the Chabad House. They shot the Holtzbergs and the visiting rabbis, took the Israeli grandmother and Mexican tourist hostage and barricaded themselves on an upper floor.

Mir told his men to try to trade the hostages for a gunman who had been captured. Mir spoke directly to the Mexican hostage, 50-year-old Norma Rabinovich, who had been preparing to move to Israel to join her adult children.

Mir soothed the sobbing woman in accented but smooth English.

"Save your energy for good days," Mir told her during the call intercepted by Indian intelligence. "If they contact right now, maybe you gonna, you know, celebrate your Sabbath with your family."

The prisoner swap failed. Mir ordered the gunman to "get rid" of Rabinovich.

"Stand her up on this side of your door," he said. "Shoot her such that the bullet goes right through her head and out the other side...Do it. I'm listening...Do it, in God's name."

The three-day siege of Mumbai left 166 dead and 308 wounded. Twenty-six of the dead were foreigners, including six Americans. The attacks inflamed tension between Pakistan and India at a time when the nuclear-armed foes were trying to improve their relationship. The repercussions complicated the U.S. battle against Islamic extremism in South Asia and thrust Lashkar into the global spotlight.

More than two years later, Mir and his victims are at the center of a wrenching national-security dilemma confronting the Obama administration. The question, simply put, is whether the larger interests of the United States in maintaining good relations with Pakistan will permit Mir and other suspects to get away with one of the most devastating terrorist attacks in recent history.

Despite the diplomatic sensitivities, administration officials say they are pursuing those responsible.

"The U.S. government is completely determined to see justice done in the case," said a senior U.S. counterterrorism official who spoke on the condition of anonymity because of pending prosecutions. "Sometimes it takes time."

For more than six months, ProPublica has examined the investigation of the attacks and previous cases documenting the rise of Lashkar. This account is based on interviews with more than two dozen law enforcement, intelligence and diplomatic officials from the United States, India, Pakistan, France, Britain, Australia and Israel, including front-line investigators. ProPublica also interviewed associates and relatives of suspects and victims, some of whom had not discussed the case with journalists, and reviewed foreign and U.S. case files, some of them previously undisclosed.

These documents and interviews paint the fullest portrait yet of Mir, a mysterious figure whose global trail traces Lashkar's evolution. His name has surfaced in investigations on four continents, his web reaching as far as suburban Virginia. Fleeting glimpses of him appear in case files and communications intercepts. A French court even convicted him in absentia in 2007. But he remains free and dangerous, according to U.S. and Indian officials.

Mir has close ties to Pakistan's security forces and may have been an officer in the powerful Inter-Services Intelligence Directorate (ISI). The ISI has been accused for years of playing a "double game": acting as a front-line U.S. ally in the fight against terror while supporting selected terrorist groups. The Mumbai case provides the first detailed inside account of how that game is played, thanks to the confession of Headley, Mir's American operative, a colorful character in a story of global intrigue.

U.S. investigators are persuaded that ISI officers recruited and trained Headley in spying techniques, then ran him as an agent in tandem with Lashkar. Pakistani military officers funded and directed Headley's reconnaissance in India, supplied tactical advice for the Mumbai attack and participated in a follow-up plot against Denmark, according to U.S. and Indian officials.

ProPublica's investigation leads to another disturbing revelation: Despite isolated voices of concern, for years the U.S. intelligence community was slow to focus on Lashkar and detect the extent of its determination to strike Western targets. During that period, Washington largely accepted Pakistan's quiet tolerance of Lashkar, which unlike its allies has not attacked the Pakistani state. Most U.S. officials admit that counterterrorism agencies grasped the dimensions of the threat only after the Mumbai attacks.

The FBI investigation into the killings of the Americans has focused on a half-dozen accused masterminds who are still at large: Mir, top Lashkar chiefs and a man thought to be a major in the ISI. U.S. officials say they have urged Islamabad to arrest those suspects.

"We put consistent pressure on the Pakistanis to deal with Lashkar and do so at the highest levels," said the senior U.S. counterterrorism official. "There has been no lack of clarity in our message."

But officials acknowledge that the response to the Mumbai attacks has been insufficient. The effort to bring to justice the masterminds - under a law that makes terrorist attacks against Americans overseas a crime - faces obstacles. A U.S. prosecution could implicate Pakistani military chiefs who, at minimum, have allowed Lashkar to operate freely. Pressure on Pakistan to confront both the military and Lashkar could damage counterterrorism efforts.

"It's a balancing act," a high-ranking U.S. law enforcement official said. "We can only push so far. It's very political. Sajid Mir is too powerful for them to go after. Too well-connected. We need the Pakistanis to go after the Taliban and al-Qaeda."

Pakistani officials told ProPublica that they have no information on Mir. They denied allegations that the ISI supports Lashkar. They point out that hundreds of ISI officers have been killed in clashes with Islamic militants.

"Allegations of ISI's cadres operating in connivance with the militants...are based on malicious intent," said a senior Pakistani official who spoke on the condition of anonymity because of the issue's sensitivity. ISI "remains top-to-bottom transparent and rests under the complete control of the civilian government...There is no question that the government thinks that all militants are enemies of the state."

Pakistan has charged Lashkar's military chief and six less-influential suspects in the Mumbai attacks. But the trial has stalled, raising fears among U.S. and Indian officials that the prosecution will collapse in a court system that rarely convicts accused extremists.

The U.S. investigation turned up 320 potential targets abroad - only 20 of them in India - including U.S., British and Indian embassies, government buildings, tourist sites and global financial centers, officials say. Many of the targets were scouted by Headley, who roamed the world on terrorist missions despite repeated warnings about him to U.S. agents from spouses and associates. The failure to stop him results partly from a lack of attention to Lashkar, experts say.

"There should have been a recognition that Lashkar had the desire and the potential to attack the West and that we needed to get up to speed on this group," said Charles Faddis, a retired CIA chief of counterterrorist operations in South Asia and other hot spots. "It was a mistake to dismiss it as just a threat to India."

Today, Mir personifies Lashkar's evolving danger. The group's longtime ties to the security forces have made it more professional and potentially more menacing than al-Qaeda. Recent intelligence shows Lashkar remains intent on striking the West and that the group increasingly blurs together with al-Qaeda and other networks in the jihadist cauldron of Pakistan, according to Western anti-terror officials.

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

Lashkar's Beginnings

For more than a decade, Sajid Mir has operated in a murky underworld of spies, soldiers and terrorists.

An Interpol notice last year seeking his arrest illustrates confusion about basic facts of his life. The Indian warrant identifies him as Sajid Majid, but most investigators still call him Sajid Mir, saying Majid may be his true name or one of several aliases. Interpol says his birthdate is Jan. 1, 1978, which would make him 32. But Headley, his star operative, told Indian investigators Mir was born in 1976, according to a 119-page report on his interrogation in Chicago last year by India's National Investigation Agency. Most investigators think he is in his mid to late thirties.

Mir's father was born in India and joined the Muslim exodus during the 1947 partition that created Pakistan, a seminal event that shaped the profound hatred of India among Pakistani militants. Mir was born in Lahore, Pakistan's cultural capital and second-largest city, but grew up in Karachi. He has two brothers and two sisters, according to the Indian report.

Mir spent time in Saudi Arabia during his youth because his father worked there. Mir became steeped in Saudi fundamentalist ideology and developed a fierce anti-Semitic streak. His family may have run a manufacturing business in Lahore, according to Australian court testimony.

Mir was a teenager when a professor named Hafiz Saeed created Lashkar-i-Taiba (the Army of the Pure) in the late 1980s with Abdullah Azzam, a Palestinian Islamist. Azzam had another claim to fame: He was an ideological mentor of Osama Bin Laden and helped him found the organization that was the forerunner of al-Qaeda.

Lashkar joined the fight against the Soviets in Afghanistan supported by the United States and Pakistan. Soon, Pakistani strategists built Lashkar into a proxy army against India in the disputed territory of Kashmir. Lashkar boomed, thanks to the funds and guidance of the Pakistani state and Saudi benefactors. The group won vast support with its mix of extremism and nationalism and its array of schools, hospitals and social programs, especially in the Punjab, Mir's home region. Indians called Lashkar "the government mujaheddin."

Mir joined Lashkar when he was about 16, investigators say. Some senior U.S., British and French anti-terrorism officials say he also spent time in the military, although that remains unclear. For years, it was common for the Pakistani military to detail officers to Lashkar, according to investigators and court testimony. The ISI is part of the armed forces.

Mir rose rapidly in Lashkar, becoming chief of its unit in Lahore and then joining the international wing, which embraced global jihad in the 1990s. Lashkar militants fought in wars in Afghanistan, Bosnia and Chechnya and built far-flung recruitment and financing networks. Those activities and Lashkar's anti-American and anti-Jewish propaganda showed an increasingly internationalist bent, according to U.S. congressional testimony and Pakistani and Western officials.

Yet the U.S. intelligence community still viewed the group as a regional player focused on India and Kashmir. Rep. Gary L. Ackerman (D-N.Y.), the former chairman of the House Foreign Affairs Subcommittee on the Middle East and South Asia, said he tried and failed to get Lashkar designated as a terrorist organization in the late 1990s.

"I said it had a huge potential for damage," Ackerman recalled. "People were not paying attention."

Lashkar trained tens of thousands of holy warriors. It was easier to join than al-Qaeda, operating openly from storefront offices across Pakistan and attracting Westerners with slick propaganda in English. Some foreign Lashkar trainees went on to join al-Qaeda; several led al-Qaeda plots against New York and London.

Mir became a deputy to the director of Lashkar's foreign operations wing. He had direct access to Zaki-ur-Rehman Lakhvi, Lashkar's military chief, and ties to al-Qaeda in neighboring Afghanistan, according to a French investigation. After the Sept. 11 attacks, Mir began grooming foreign volunteers who had come to Pakistan to wage war on the West.

Willie Brigitte became one of Mir's favorites. Born in Guadeloupe and radicalized in Paris, the Afro-Caribbean convert was dour, burly and nearsighted behind round-rimmed glasses. Fellow trainees called him "the Grouchy Frenchman."

Brigitte was part of an al-Qaeda-connected group of militants in Europe involved in numerous plots. In September 2001, he set off for Pakistan hoping to reach the Afghan battleground. He made his way to Lashkar headquarters in Muridke outside Lahore. The complex featured a mosque, a university, dormitories and houses for leaders. Brigitte briefly studied Arabic and the Koran and met Mir, the coordinator of foreign recruits, who carried himself like a rising star.

"He was in fact an important personage," Brigitte testified later in France. "He was a man of about 30, very cordial and pleasant, with whom I had a good relationship."

Of medium build, Mir had a dark complexion, black hair and a thick beard. He spoke English, Urdu, Hindi and Arabic. His nicknames were Abu Bara (Father of Bara), Uncle Bill, Sajid Bill, Wassi and Ibrahim. A Makarov pistol on his hip, he was accompanied by two bodyguards and a driver, according to Brigitte's testimony. Mir was secretive, meticulous and adept with computers, according to the accounts of several recruits who have been captured. He was also charming, manipulative and ruthless.

His recruits included four militants from the Virginia suburbs of Washington, D.C. They were part of a multiethnic crew of college graduates, U.S. Army veterans and gun enthusiasts whose spiritual leader was Ali Al-Timimi, an Iraqi-American