New Delhi wants to interrogate David Headley, who this week admitted to a US court to scouting targets for the 2008 rampage on India's financial hub, which killed 166 people and derailed peace dialogue between India and Pakistan.
US Assistant Secretary of State Robert Blake, on a two-day trip, assured authorities of cooperation in the Headley case.
"The other question that has been raised is whether Indian investigators will be allowed access to Headley to learn more about his involvement in the planning of the Mumbai attacks, and the answer to that is yes," Blake told reporters.
Headley has been cooperating with US investigators since his arrest in October and faces up to life in prison. He has pleaded to 12 counts, including conspiring to bomb and murder US and Indian citizens.
In an agreement with prosecutors, Headley promised to help investigators and give testimony against others in exchange for a pledge he would not be extradited to India, Pakistan or Denmark.
India's Home Minister has said India still had "many more questions" about the Chicago man, who spent his childhood in Pakistan, and would not withdraw a request for extradition.
Blake did not rule out such a move at a future date if fresh charges were brought against Headley, which would stand outside the plea bargain.
Blake also said it was very important for Pakistan to tackle the threat of the Lashkar-e-Taiba (LeT), the Pakistan-based militant outfit whom India blames for Mumbai and whom Headley pleaded guilty to supporting.
"(LeT) is a growing concern to the United States, because of the increasing global scope and ambition of LeT," Blake said.
