By 2000 he was back in Afghanistan serving as an instructor at a training camp near Kabul, where he taught about explosives, artillery and topography, according to the file. Shadi Abdalla, a former Bin Laden bodyguard, described him in later testimony as 5 foot 7, muscular and tanned, with graying black hair and a graying beard.
During the U.S.-led military operation in Afghanistan in late 2001, Masri fought in the 055 Brigade, a paramilitary unit that took heavy casualties covering Bin Laden's escape into Pakistan, according to Rohan Gunaratna, author of "Inside Al Qaeda." Holed up in the border region, the survivors split into two wings, he said. Internal operations ran combat in Pakistan and Afghanistan, where Masri helped the Taliban regroup. External operations oversaw attacks elsewhere.
After the capture in 2003 of the manically prolific Khalid Shaikh Mohammed, the self-proclaimed Sept. 11 mastermind, Masri's duties shifted. He joined a group of chiefs who tried to keep targeting the West, mainly Britain.
They succeeded on July 7, 2005, killing 52 people in synchronized public transportation bombings in London. The lead bombers were groomed in Pakistan by Abdul Hadi, a former Iraqi military officer, according to officials and court testimony. Masri's name emerged as a planner working with him.
"He's considered a player," a U.S. anti-terrorism official said. "He comes up on the radar screen a few months after July 2005."
In January 2006, an airstrike killed 18 people in the Bajaur region of Pakistan. The press reported, inaccurately, that Masri and three other leaders died in the rubble.
The plane plan
Masri had embarked on his biggest task yet: a mega-project intended to match the carnage of the Sept. 11 attacks by blowing up airplanes en route from Britain to the United States. Half a dozen British militants traveled to Pakistan for training.
"He was involved in recruiting, overseeing the lesson plan, so to speak," the U.S. anti-terrorism official said.
The innovative techniques required special instruction. Masri envisioned his operatives injecting the liquid explosives, a highly concentrated hydrogen peroxide mix, with a syringe into the false bottoms of innocuous containers such as sports drinks, sneaking the components aboard and assembling bombs after takeoff.
"The airline plot is his thing," a Western intelligence official said. "And it is a major plot."
Investigators think only one or two trainees had contact with Masri. Trainees had autonomy to instruct and supervise a dozen fellow militants back in Britain, officials say. In turn, investigators believe Masri got direction from his bosses, who often communicate with the command structure through messengers.
"We have patchy intelligence on the relationship and structure between external operations figures and Zawahiri and Bin Laden," the senior British official said. "In the really big plots, we think they played a role."
Investigators monitored the plotters for months, managing to film inside their London safe house. In August 2006, police rounded them up. The attack was weeks away and would have targeted five planes, the U.S. anti-terrorism official said.
Three months later, Pakistani helicopter gunships blew up a remote madrasa, killing about 80 people but missing Masri, officials say. In late 2006, however, Israeli, Turkish and U.S. spies teamed up to capture Hadi, the former Iraqi military officer, in Turkey as he was en route to Iraq to improve relations with the Al Qaeda offshoot there, officials say.
Masri assumed more control. He allegedly turned his aim to another part of Europe he knows. Last spring, he taught bomb-making in compounds in North Waziristan to aspiring suicide attackers, including a 21-year-old Pakistani living in Denmark and a 45-year-old Pakistani-German, according to U.S. and European officials.
A U.S. anti-terrorism source sees Masri's role as a symptom of decline. "The fact he trained them himself shows you some of the limitations of the network," the source said.
In any case, Masri's pupils apparently displayed more fervor than stealth. Aided by U.S. intercepts of communications to Pakistan, Danish police put the 21-year-old under surveillance along with his associates, one of whom had been in Pakistan at the same time. As in London, police got deep inside the alleged cell, U.S. and European officials say.
Police installed clandestine cameras and microphones in the 21-year-old's apartment in a scruffy area that mixes immigrant families and young Danes. In early September, the cameras filmed the 21-year-old and an Afghan suspect as they sang militant songs and mixed TATP, the explosive used in the London attacks. The two even conducted tests of detonators in a vestibule, officials say.