A new ProPublica investigation into the November 2008 Mumbai terrorist massacre raises troubling questions about Pakistan's dual roles as an American ally and a supporter of jihadist terror. The three-day siege ended with 166 people and 308 wounded. Six Americans were among the 26 foreigners killed.

Veteran journalist Sebastian Rotella documents in detail the close relationship between the perpetrators of the massacre- the Kashmiri separatist group Lashkar-e-Tayyiba – and the Pakistani government.

"The question, simply put, is whether the larger interests of the United States in maintaining good relations with Pakistan will permit [jihadist] suspects to get away with one of the most devastating terrorist attacks in recent history," he writes.

Rotella explains the problem by examining the actions of Sajid Mir, the terrorist operative who directed the Mumbai slaughter by telephone from a safe house in Karachi. Mir spent two years conducting reconnaissance of terrorist targets using a Pakistani-American businessman named David Coleman Headley.

Through interviews with law enforcement and intelligence officials in the United States, India, Pakistan and elsewhere, ProPublica provides what may be the most detailed account yet of Mir's activities and his connections with Pakistan's Inter-Service 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," Rotella writes.

U.S. officials emphasize that they are constantly pressuring Pakistan to act against Lashkar. But they say they are limited in their ability to press Islamabad to go after Mir – a terrorist with American blood on his hands. "Sajid Mir is too powerful for them to go after. Too well-connected," a high-ranking U.S. counterterrorism official tells Rotella. "We need the Pakistanis to go after the Taliban and al-Qaida."

Read the full ProPublica report here.