Underpants bomb threat to flights from UK John Brennan, the White House counter-terrorism adviser, told ABC News that he could not rule out that other bombers are still at large. Every flight from Gatwick was carrying armed officers, according to one report. The Department for Transport refused to comment last night. Around 1,000 US air marshals operate in teams of two or three and undergo intense firearms training to allow them to open fire on a crowded aircraft. Hillary Clinton, the US secretary of state, said the latest plot indicated that terrorists "keep trying to devise more and more perverse and terrible ways to kill innocent people." The Saudis have previously cracked a plot by al-Asiri to blow up airliners, including one travelling through Leicestershire, using devices disguised as desk-top printers. The latest underpants device was handed over two weeks ago and President Barack Obama was informed as the CIA then targeted those who gave it to the courier. On Sunday, a senior planner with AQAP called Fahd al-Quso was hit by a missile fired from an unmanned drone as he stepped out of his vehicle in Rafd, a mountain valley in the southern Yemeni province of Shabwa. Quso, 37, who had a $5 million bounty on his head, was indicted in the US for his role in the bombing of the USS Cole in Yemen in 2000, which killed 17 American sailors. He was also one of the most senior al-Qaeda leaders to meet Umar Farouk Abdulmutallab, the Detroit underpants bomber, in Yemen before he left the country to launch his attack. Al-Asiri is one of the most wanted terrorist leaders in the world. The son of a former officer in the Saudi armed forces, he has been described by US intelligence officials as the most "ruthless and fanatical of all al-Qaeda's followers." He recruited his brother, Abdullah, to join al-Qaeda, and in one of his most chilling attacks, sent him back to Saudi Arabia for a meeting with Muhammad bin Nayef, the security minister. Abdullah blew himself up using a prototype underpants bomb, but left the minister with only minor injuries.