WASHINGTON US spy agencies in recent months picked up clues pointing to an Al Qaeda attack out of Yemen and were moving to disrupt it, but a crucial piece of information fell through the cracks.

Intelligence officials describe a trail of warning signs for the botched Christmas Day attack on a US-bound airliner dating back to August, when the National Security Agency reportedly intercepted chatter among Al Qaeda operatives in Yemen.

The NSA, which runs an elaborate global eavesdropping operation, heard conversations from Al Qaeda figures describing a plot to recruit a Nigerian man for a terrorist attack, the New York Times reported.

The discussions about a possible attack coincided with an alarming demonstration of Al Qaeda's growing strength in Yemen.

The same month, a bomber crossed from Yemen and staged a suicide attack against Saudi Arabia's anti-terror chief, Prince Mohammed bin Nayef.

The attack failed but Saudi officials in October reportedly told President Barack Obama's top counter-terrorism adviser that the bomber had sewn explosives into his underwear, the same tactic used in the Christmas Day plot.

In September, the head of the National Counterterrorism Centre, Michael Leiter, warned a senate hearing that Al Qaeda had gained a dangerous foothold in Yemen and was turning it into a regional base for operations.

Sometime in October or November, Alhaji Umaru Mutallab - the father of the Nigerian man accused of trying to blow up Northwest Flight 253 - approached US embassy officials in Abuja and told them he was worried his son had become radicalised by extremists in Yemen, the Times wrote.

His son's name, Umar Farouk Abdulmutallab, was passed on to intelligence agencies - including the National Counterterrorism Centre - and entered into a vast data base of individuals with suspected links to militants or terrorist groups, officials said.

But as the father apparently gave no indication that his son planned an attack, US authorities did not add Abdulmutallab's name to a higher level terror watch list or to the “No Fly List” designed to prevent suspected terrorists from boarding aircraft.

Moreover, State Department officials remained unaware that Abdulmutallab had a valid US visa due to a misspelling of his name, and therefore did not review his visa status.

By mid-December, senior intelligence officials - hoping to derail a possible attack in the works - reportedly ordered two waves of missile strikes against Al Qaeda training camps in Yemen.

But they remained unaware of the pivotal information about Abdulmutallab that might have foiled the plot, which was now at the final stage.

At about the same time Al Qaeda targets were coming under missile fire in Yemen, Abdulmutallab reportedly purchased a flight ticket with cash in Ghana.

There are conflicting reports if his ticket was one-way or round-trip.

US border security officials had planned to question Abdulmutallab upon his arrival in Detroit as part of standard procedure because his name appeared on a central data base that lists half a million individuals, said an administration official, who spoke on condition of anonymity.

“As we have indicated before, there were bits and pieces of information about Abdulmutallab available in a variety of areas in the system prior to December 25,” he told AFP.

But US authorities had no plans to boot him off the plane taking off from Amsterdam as his name had not been added to the more critical terrorist watch lists, he said.

Former and current intelligence officials say the problem was not a lack of information but a failure to manage it, with the spy bureaucracy overlooking the link between the “chatter” from Al Qaeda and the concerns raised over Abdulmutallab.

Intelligence work “is not a science, it's an art”, said former CIA officer Bruce Riedel.

“In retrospect it's always simple. You just look at a couple of dots that all made sense from the beginning and it all adds up. But in the real world, it's never that simple.”—AFP