The four member NIA team has concluded questioning of Mumbai terror suspect David Coleman Headley and has headed back home, Indian ambassador to the US Meera Shankar said today.

"This (the questioning) took place over several days, beginning from June 3, 2010, and the team is now returning to India on the conclusion of a useful visit," Shankar told PTI in response to query on Headley's questioning by a team of Indian investigators.

"The government of India attaches importance to this in investigating the full dimension of this heinous act of terrorist violence. The support and cooperation extended by the US authorities is appreciated and is in keeping with the commitment of the two countries to strengthen their cooperation in meeting the challenge of terrorism," Shankar said.

Headley had pleaded guilty to conspiring in the 26/11 Mumbai attacks but struck a deal with US authorities in a plea bargain that saved him from the death penalty and extradition to India.

The plea agreement had however said Headley would cooperate with foreign authorities and can be interviewed by them only on US soil.

India had asked the US to grant access to the 49-year-old Headley, being held in the federal lock-up Metropolitan Correctional Centre in Chicago.

"Following discussions with US Attorney General, Eric Holder, the team from the NIA has visited the US for questioning Headley," Shankar said.

The National Investigation Agency (NIA) team, led by Loknath Behera, questioned Pakistani-American Headley face-to-face, eliciting information about his role in the Mumbai attacks, the wider conspiracy and all those involved in the carnage, sources had earlier said.

The interrogation of Headley, who is accused of having conducted recce for the Mumbai attacks, is expected to shed more light on the LeT's plans regarding terror attacks in India.

The questioning revolved around the places he had visited in run up to the Mumbai terror attacks of November 2008 and the people he had been in touch with.

Headley's interrogation by Indian sleuths was facilitated by his plea bargain with the US government under which he expressed readiness to be subjected to questioning by foreign investigators.