David Headley pleaded guilty in March to a dozen federal terrorism charges related to the Mumbai attacks as well as to a plot to attack a Danish newspaper that had published the Prophet’s (pbuh) cartoons in 2005.

“There were no restrictions on the questions posed by Indian investigators,” the Justice Department said. The interviews started on June 3 and lasted until Wednesday, according to US officials.

“To protect the confidentiality of the investigations being conducted by both India and the United States, both countries have agreed not to disclose the contents of the interviews,” the department said.

Headley, who helped the militant group Lashkar-e-Taiba (LeT) plot the Mumbai attacks, had agreed to help investigators and give testimony against others in exchange for a promise that he would not be extradited to India, Pakistan or Denmark. Indian officials had pressed for access to Headley, noting that while he had provided significant amounts of information they still had further questions. Headley spent his childhood in Pakistan and his father is Pakistani.

Headley was arrested at Chicago O’Hare International Airport in October before leaving for Pakistan. He was found with about a dozen surveillance videos from Denmark that he had planned to deliver to his handlers. The US ambassador to India said in a statement that the access given to the investigators highlighted the close cooperation between the two countries on terrorism matters. “The Headley interviews were historic in the nature of security cooperation. This strategic partnership is significant, substantive, and highly successful,” Ambassador Timothy Roemer said.

Headley has not yet been sentenced but faces up to life in prison. The 2008 attacks in Mumbai, in which six Americans were among the dead, lasted for three days and further escalated tensions between India and neighboring Pakistan where LeT militants are based.