WASHINGTON - The United States is confident the top al Qaeda bomb maker, believed to be the mastermind behind a failed bombing of a US-bound airliner in 2009, has been killed, two US officials said even as others cautioned the evidence was not conclusive.Washington has long sought Ibrahim Hassan al-Asiri, a militant with al Qaeda's Yemen branch who is one of the world's most feared bomb makers because of his ability to create hard-to-detect bombs, including some implanted in suicide bombers.Two US officials, including a senior official, said they were confident al-Asiri had been killed. They were speaking on the condition of anonymity."We are pretty confident that he has been killed," one of the officials said.That official added, however, that the usual fallout from a senior militant being killed, like a eulogy from Yemen's al Qaeda branch, known as Al Qaeda in the Arabian Peninsula (AQAP), had not appeared. Separate US intelligence officials said they do not consider the available evidence conclusive.A report by UN experts monitoring Islamic State and al Qaeda for the UN Security Council, which was made public last week, said some states had reported that al-Asiri "may have been killed during the second half of 2017.""Given al-Asiri's past role in plots against aviation, this would represent a serious blow to operational capability," the UN experts wrote.US officials cautioned that while his death would be a major symbolic blow to AQAP, al-Asiri's bomb-making skills almost certainly have been passed to others, and the threat from the group does not appear to have been significantly reduced.