US investigators have zeroed in on a Pakistani national suspected to be a key link between Lashkar-e-Toiba (LeT) operatives David Coleman Headley and Tahawwur Hussain Rana - allegedly involved in the Mumbai attacks - and LeT handlers like Zaki-ur Lakhvi.

The Pakistani national, whose identity has been kept secret, is believed to have been in Pakistan at the time of November attacks in Mumbai.

India and the US are in constant touch on the Headley case and Washington has conveyed that within a week there could be "authentic" information on whether the duo were involved in the 26/11 attacks, sources said.

Headley, a Pakistan-born US citizen, and Rana, Pak-born Canadian citizen, were arrested last month by FBI for allegedly plotting terror attacks in India and Denmark. Indian investigators have found that the duo had visited India a number of times and toured various places apparently to conduct recce.

The US believes that some elements of the ISI could have links with Headley. This belief is based on the arrest of "two key persons" in Pakistan, sources said. Illyas Kashmiri, a former Pakistani military officer who has become a militant commander associated with both al-Qaeda and LeT, has been detained in Pakistan.