Indian investigators have conducted extensive interviews with a Chicago man who has pleaded guilty to scouting targets for the 2008 Mumbai attacks.
David Coleman Headley answered questions over seven days, the US justice department said in a statement.
He entered a guilty plea on 12 counts in a Chicago court in March. More than 170 people died in the Mumbai attacks.
He also admitted plotting to attack a Danish newspaper that published a cartoon many Muslims deemed offensive.
Headley, a Pakistani-American, initially denied the charges but changed his plea to avoid the death penalty or extradition to India, Pakistan or Denmark.
Correspondents say that it is believed to be one of the first times that Indian officials have been granted permission to interrogate a criminal suspect in the US.
Indian law enforcement officials were provided direct access to interview Headley "as part of the co-operation and partnership between the United States and India in the fight against international terrorism," the justice department said.
"Headley and his counsel agreed to the meetings and Headley answered the Indian investigators' questions.
"There were no restrictions on the questions posed by Indian investigators," it said.
Headley, a Pakistani-American, made several surveillance trips to India and Denmark.
According to court documents, he passed on information to his contacts within the Pakistan-based Islamic militant group, Lashkar-e-Taiba.
The group has been blamed for organising the Mumbai attacks.
Headley was arrested by FBI agents in Chicago in October while trying to board a plane for Philadelphia.
He is alleged to have told prosecutors that he had been working with Lashkar-e-Taiba since 2002.
He was first charged with plotting to attack the Danish newspaper Jyllands-Posten after they angered Muslims by publishing cartoons of the Prophet Muhammad.
Headley changed his name from Daood Gilani in 2006 after he was told by members of Lashkar-e-Taiba that he would be travelling to India to carry out surveillance duties for the group, prosecutors said.