The United States has asked Pakistan for help investigating a failed plot to bomb New York's Times Square, The Washington Post reported on Thursday, as investigators kept questioning a Pakistani-American charged in the case.

Washington was preparing a detailed request for urgent and specific assistance in the investigation to be presented by the end of the week, the Post reported, quoting an Obama administration official.

So far, the United States had only asked to interview the parents of the suspect, the newspaper said, quoting a Pakistani official as saying the parents had not been located.

US investigators see credible signs the Pakistani Taliban movement had links to the the suspect, Faisal Shahzad, in the attempted car bombing, US officials said.

US investigators were taking 'a hard look' at the possible links between the suspect and Tehrik-e-Taliban Pakistan, or TTP, suggesting there was a growing body of intelligence pointing in that direction, a US official said on Wednesday.

'TTP is entirely plausible but we're not ruling out other groups,' the official said, speaking on condition of anonymity.

'It's important to develop a complete intelligence picture before reaching any final conclusion, but all the brush strokes aren't there yet,' the official said.

In a sign of heightened security in America's most populous city since the failed bomb attempt on Saturday, authorities shut down New York's busy Triborough Bridge on Wednesday night after a rental truck was found abandoned and smelling of gas.

No bomb was found in the truck and police said the bridge would reopen shortly.

Shahzad, who US prosecutors say admitted driving a car bomb into Times Square waived his right to an initial court appearance on Wednesday to keep talking to investigators, sources said.

Shahzad, 30, who was born in Pakistan and became a US citizen last year, has been charged with attempting to use a weapon of mass destruction and trying to kill and maim people within the United States as well as other counts.

The Taliban in Pakistan has claimed responsibility for the plot.

CIA-operated drones have targeted Taliban figures in Pakistan's tribal areas and the group has vowed to avenge missile strikes that have killed some of its leaders. If confirmed that the group sponsored the failed bombing in New York, it would be their first attack on US soil.

US prosecutors said Shahzad, the son of a retired Pakistani vice air marshal, had admitted to receiving bomb-making training in a Taliban and al Qaeda stronghold in Pakistan. A law enforcement source said investigators believed the Pakistani Taliban financed that training.

Street vendors alerted police to a smoking vehicle parked awkwardly in Times Square on Saturday evening with its engine running and hazard lights flashing. Thousands of people were evacuated and a police bomb squad defused the crude device, which included firecrackers and propane gas tanks.

'I have been expecting you'

Shahzad was arrested on Monday night after he was removed from Emirates plane at New York's John F. Kennedy International Airport that was about to depart for Dubai. He had been on his way back to Pakistan.

'I was expecting you. Are you NYPD or FBI?' local media reported Shahzad as saying to Customs and Border Protection agents when they approached him on the plane.

Shahzad waived his right to an initial court appearance within 48 hours of his arrest and other US constitutional rights, a US official and sources said. He faces life in prison if convicted of the charges against him, unless he negotiates a lesser sentence in exchange for cooperation.

'(Shahzad) was giving them intricate details as to what he did overseas,' said a law enforcement source familiar with the investigation. 'There was a determination that there wasn't anyone else in the (New York) area to target.'

Shahzad bought a ticket with cash and boarded the plane even though he had been placed on a US 'no-fly' list earlier that day. On Wednesday, the Obama administration ordered airlines to step up their efforts to prevent people on the list from boarding flights.

Several of Shahzad's relatives were arrested in Pakistan after he was taken from the plane, Pakistani officials said.

Shahzad, a former budget analyst who worked for a marketing firm in Connecticut, came from a relatively privileged background that offered no hints of radicalism.

Residents of his home village of Mohib Banda were in disbelief. 'I never observed any inclination for militancy,' a close family friend told Reuters. – Reuters