WASHINGTON (CNN) -- Abu Ubaida al-Masri, a senior al Qaeda commander suspected of helping plan the 2005 London subway bombings, died of natural causes in Pakistan, U.S. counterterrorism officials said Wednesday.
Al-Masri was considered an explosives expert and one of the terrorist network's top figures, with close ties to Osama bin Laden and his deputy, Ayman al-Zawahiri. One U.S. official described him as a "very bad, bad guy."
According to the official, al-Masri died several months ago in Pakistan, possibly from hepatitis.
A Pentagon source, speaking on condition of anonymity, said military officials had "no reason to doubt" reports that al-Masri had died.
Officials described him as a longtime jihadist who first fought against the Soviet occupation of Afghanistan in the 1980s. He was also believed to have fought in Bosnia and Chechnya in the 1990s before returning to Afghanistan, where al Qaeda was based before its September 11, 2001, attacks on New York and Washington. Watch a profile of al-Masri from the CNN archives »
Al-Masri fled Afghanistan after the U.S. invasion that followed those attacks, but continued to operate along the mountainous border region with Pakistan. Officials said he helped plan external attacks and train jihadists to carry out those plots.
The officials said al-Masri played a "strong hand" in planning the London attacks in July 2005, when synchronized bombs in subways and buses in the British capital killed 52 people.
And he played a key role in what would have been a far more spectacular attack -- a plot to simultaneously bomb trans-Atlantic passenger jets. British authorities foiled the plan in its late stages in 2006.
CNN's Jamie McIntyre contributed to this report.