''NIA has been entrusted with the investigation into both David Headley and Rana. They are continuing with it and we expect that investigations will take about four to six weeks and as soon as investigations are completed, the NIA will file the necessary chargesheet,'' Home Secretary G K Pillai said.

He was talking to reporters on the sidelines of a DRDO seminar on low-intensity conflict.

Headley, a Pakistan-born US citizen and Rana, a Pakistani Canadian, were arrested by the FBI in Chicago in October on charges of plotting terror attacks in India at the behest of Lashkar-e-Taiba.

On cooperation with the FBI, Pillai said the American agency had shared with India information given to it by Headley.

"We have given them a supplementary list of questions," he said while hoping they would respond to them.

Answers to some of the Indian queries would be sought by the FBI from Pakistani authorities also, he said.

"FBI team has gone to Pakistan and they would be seeking some answers there since Headley had visited that country with some of the LeT handlers," Pillai said.

Asked if India would seek access from the United States to David Headley, Pillai said, "once we file the chargesheet, we will definitely be seeking access both for interrogation and a subsequent stage for his extradition."

In response to a query on involvement of more Pakistani serving and retired Army personnel in the 26/11 attacks, the Home Secretary said, "there is some evidence, which we hope that when the FBI team goes to Pakistan, they would be able to corroborate some of the information that they have shared with us and subsequently they would share that also with us."