Jamil Ahmed al-Badawi, the mastermind behind the al Qaeda attack on the U.S.S. Cole in October 2000, was among the 23 escapees from a Yemen jail last week.
"He was the most valuable al Qaeda member in prison in Yemen, and yet he escaped twice," said Richard Clarke, a former White House counterterrorism adviser and now an ABC News consultant.
Badawi, whose death sentence was commuted to 15 years in prison, escaped once before, in 2003. At the time of this escape, he and the 22 other escapees were held by Yemen's version of the CIA, which is located in the capital city of Sanaa.
Officials say someone dug a 460-foot tunnel from the women's section of a mosque into the basement prison cells of the public-security organization. According to engineers, that effort would take at least a month and would mean removing at least three dump-truck loads of dirt.
"The problem with the public-security organization is it's just so incredibly unreliable," said former FBI agent Jack Cloonan, who is now an ABC News consultant. "There is no way this could happen without someone on the inside giving some assistance."
Badawi was similarly meticulous in his organizing of the bombing on the U.S.S Cole, spending more than a year planning it. Seventeen U.S. sailors were killed in that attack.
The father of one of those victims said he was disgusted by Badawi's escape.
"We're just sick and tired of it. Every time they catch somebody, they end up getting away," said Lou Gunn, whose son, Seaman Cherone L. Gunn, died in the attack. "We're not getting the justice that we deserve."
U.S. officials say they believe that Badawi and the other escapees are likely to head for the rugged mountains of the Hadhramaut region on the border between Yemen and Saudi Arabia, the ancestral home of al Qaeda leader Osama bin Laden.
"Operating from Hadhramaut," said Clarke, "he can bring together al Qaeda cells from around the country and from outside the country. And he can plot attacks as al Qaeda has done in the past from their secret bases in Hadhramaut."
ABC News' Avni Patel contributed to this report.