As evidence mounted of involvement of Pakistani Taliban in the Times Square bombing plot, with reports that the suspect had meetings with the group's bomb-maker Qari Hussain, US investigators have now shifted focus to tracing the links to the volatile Af-Pak border.
The US probe is now focussing on unearthing the Taliban and other militant links to the plot following revelations by Faisal Shahzad that he attended a terrorist training camp in Waziristan, a stronghold of Taliban and al-Qaeda militants.
"A big part of that ongoing investigation is to evaluate where he was and what he was doing during his time in Pakistan," White House spokesman Robert Gibbs told reporters.
Gibbs said Justice Department and investigating agencies are actively looking at the time which Shahzad, son of a retired Pakistani Air Vice Marshal, spent in Pakistan and New York Police Commissioner Raymond Kelly said a team was dispatched to Karachi, where several people have been arrested in connection with the case.
US President Barack Obama met his National Security Team which includes, Secretary of State Hillary Clinton and Defence Secretary Robert Gates to discuss in depth the Pakistani links to terror attacks in America.
The US Ambassador to Islamabad Anne Patterson has held discussions with President Asif Ali Zardari and top Pakistani cabinet ministers about the search for leads in the US investigations.
The Obama Administration is preparing to push Pakistan for harder action against dreaded outfits like LeT, Taliban, al-Qaeda and Haqqani network, fearing that these groups are aiming to spread their terror footprints to the country, US media reports said.
Washington Post said that US Administration was preparing to deliver to Pakistan detailed request for "urgent and specific" assistance on the case by the end of the week.
The main fear rankling the US investigators is whether Shahzad acted alone, had outside help or was acting on behalf of a larger group either in Pakistan or elsewhere.
In his last trip to Pakistan, Fox News quoting Pakistani officials said that Shahzad was introduced to Qari Hussain, the Number 3 in the top tier leadership of the Pakistani Taliban, the organisation's bomb maker and suicide trainer.
The New York Times quoted officials as saying there were strong indications that Shahzad knew other key members of Pakistani Taliban and the group had a role in training him.
According to charges filed in the court, Shahzad has admitted to receiving bomb making training in Waziristan.