Who is alleged al-Qaeda underwear bomb-maker?

His attempts on western aviation have sown fear and prompted long delays, with security officials scrambling to keep up with his evolving tactics.

Ibrahim Hassan al-Asiri, the man alleged to be behind another so-called underwear bomb, has shown tactical cunning, with plots attributed to him repeatedly turning up weak points in counter-terror procedures. He is described as devout and ruthless, once sending his own brother on a daring suicide mission against a senior Saudi security chief.

On Monday, security officials fingered Mr. al-Asiri – or possibly one of his protégé, as there are conflicting reports over whether the bomb-maker was killed in the fall – over an alleged plot to bomb a western-bound airliner. The bomb is alleged to resemble his earlier work but shows a new level of sophistication.

A Yemeni believed to be in his early 30s, Mr. al-Asiri is the son of a retired soldier. He operates with al-Qaeda in the Arabian Peninsula, an offshoot of Osama bin Laden's al-Qaeda, which has taken advantage of Yemen's deteriorating security situation. The country has been the source of a number of plots targeting the west.

Western security officials blame several of the plots on Mr. al-Asiri, and their effects have been felt throughout western aviation.

The first underwear bombing attempt, on Christmas Day 2009, helped spur the move toward full-body scanners at airports. A plot involving bombs hidden in printer cartridges forced changes to cargo screening. New chemical analysis of passengers and luggage was prompted by the use in both cases of explosive pentaerythritol tetranitrate (PETN), which can elude sniffer dogs.

But it was another attempt, not aimed at aviation, that both laid bare the shortcomings of current security defences and indicated the lengths to which Mr. al-Asiri would go.

In 2009, the alleged bomb-maker's younger brother made overtures to Saudi intelligence, saying he wanted to be rehabilitated. He asked to turn himself in directly to Prince Mohammad bin Nayaf, the head of counter-terrorism and reportedly bypassed security as a mark of trust. A bomb hidden in his rectum detonated prematurely, killing him but wounding his target only slightly.

"Although the assassination attempt failed, the brutality, novelty and sophistication of the plot is illustrative of the threat posed by al-Asiri," the U.S. State Department said in a statement early in 2011, announcing new measures against both the bomb-maker and his group.

U.S. intelligence sources indicated last fall that Mr. al-Asiri had been killed in a drone attack. But Yemeni officials later countered that.