The Egyptian militant is thought to have died of Hepatitis C, a US anti-terror official said. Al-Masri was the powerful, if little-known, chief of the network’s external operations who allegedly trained recruits in hideouts in Pakistan and dispatched them on attacks against the West, according to Western investigators.

Officials in three countries believed that al-Masri had died, but investigators did not have confirmation and noted that al-Qaeda had not paid tribute to al-Masri with eulogies on the Internet as with other fallen leaders. Recently, however, anti-terror investigators detected conversations among al-Qaeda militants revealing that al-Masri had died of Hepatitis C, the US official said. Death of natural causes would explain the lack of eulogies, which are generally reserved for extremists who die violently as “martyrs,” officials said.

It is likely al-Masri has already been replaced, experts said. Potential successors include Khaled Habib, an Egyptian who—according to expert Rohan Gunaratna, author of Inside Al Qaeda—has overseen al-Qaeda “internal” operations in Pakistan and Afghanistan. Others who worked with al-Masri and may have replaced him include Hamza al Jawfi, a Gulf Arab, and Midhat Mursi, 53, an Egyptian chemist who has allegedly overseen al-Qaida’s efforts to develop unconventional weapons.

Anti-terrorism officials consider operations chiefs more urgent prey than even Osama bin Laden because they are front-line figures in attacks on the West. Al-Masri’s real name was unknown. He was an explosives expert in his mid-40s and a veteran of combat in Afghanistan, Bosnia and Chechnya, according to court files. Captured militants described him as a stocky, graying Egyptian who had lost two fingers in combat or handling explosives. He served as a trainer at a bin Laden camp and, after the Sept. 11 attacks, led paramilitary forces in the Kunar province of Afghanistan.

About four years ago, al-Masri became external operations boss after predecessors were killed or captured, officials say. He allegedly took part in planning and training for the attacks on the London transport system in July 2005.