Sources said Islamabad is moving slowly on letting US investigators on its soil but Washington has made it clear that its domestic laws require FBI to fully investigate any case where US citizens had been killed and so it would need to go to Pakistan as the next step.

Six US nationals were killed in the attacks.

As reported first by 'The Sunday Express' on December 28, US had specifically asked access from Pakistan for Lashkar operations head Zaki-ur-Rehman Lakhvi and communications in-charge Zarar Shah. The FBI has enough from forensic investigation involving telephonic conversations between the terrorists and these two LeT operatives.

In fact, Shah was the first name that US Secretary of State Condoleezza Rice mentioned to Indian leaders quite early during her visit to New Delhi in early December. She is said to have even taken it up strongly with Pakistan, demanding that his arrest was vital. Today The Wall Street Journal reported that Shah has confessed to Pakistani investigators of his role in the attacks, lending the first confirmation to India’s claims.

The rich evidence that the US is now making the basis for moving forward on the investigative front, sources said, is a result of very close Indo-US cooperation that saw FBI getting access at every level to evidence collected from the Mumbai terror sites including the satellite phone used by the terrorists.

It's learnt that FBI using its sophisticated equipment was able to decipher a lot of information from the phone that has given vital leads to take forward the investigation in Pakistan. Given that the phone had been operational several days before the terrorists left for India, the investigations in Pakistan could lead to more conclusive results.

It may be recalled that the terrorists used the phone on November 18-19 to contact Abu Saad, a local commander in Muzaffarabad who reports directly to Yusuf Muzamil, the head of LeT operations in India and a Lakhvi confidante. He is also said to be Lakhvi's right hand man, said sources. This intercept was picked up by intelligence agencies, which confirmed the boat to be a LeT vessel.

That the investigators have stitched up the entire sequence and have, at least, been able to identify the immediate handlers in Pakistan was further confirmed by reports in the Dawn newspaper which reported US passing on a taped conversation between Lakhvi and the terrorists.

Zarar Shah is also identified in several conversations that terrorists had with their Pakistani handlers while being holed up in the Taj Mahal Hotel, Oberoi and Nariman House. On some occasions, the terrorists even sought specific direction before killing foreigners from different nationalities. In some cases, the message from the other side conveyed elation and "sadistic" demands to keep the phone on while shooting. Some of the details available with investigators, sources said, are chilling and horrific.

Meanwhile, US Assistant Secretary of State for South Asia Richard Boucher is expected to visit India next week to assess the political situation and ensure better cooperation from Pakistan while asking India to show “more patience”.