In Gao, residents routinely see Moktar Belmoktar, the one-eyed emir of the al-Qaida-linked cell that grabbed Fowler in 2008. Belmoktar, a native Algerian, traveled to Afghanistan in the 1980s and trained in Osama bin Laden's camp in Jalalabad, according to research by the Jamestown Foundation. His lieutenant Oumar Ould Hamaha, whom Fowler identified as one of his captors, brushed off questions about the tunnels and caves but said the fighters are prepared.
"We consider this land our land. It's an Islamic territory," he said, reached by telephone in an undisclosed location. "Right now our field of operation is Mali. If they bomb us, we are going to hit back everywhere."
He added that the threat of military intervention has helped recruit new fighters, including from Western countries.
In December, two U.S. citizens from Alabama were arrested on terrorism charges, accused of planning to fly to Morocco and travel by land to Mali to wage jihad, or holy war. Two French nationals have also been detained on suspicion of trying to travel to northern Mali to join the Islamists. Hamaha himself said he spent a month in France preaching his fundamentalist version of Islam in Parisian mosques after receiving a visa for all European Union countries in 2001.
Hamaha indicated the Islamists have inherited stores of Russian-made arms from former Malian army bases, as well as from the arsenal of toppled Libyan leader Moammar Gadhafi, a claim that military experts have confirmed.
Those weapons include the SA-7 and SA-2 surface-to-air missiles, according to Hamaha, which can shoot down aircrafts. His claim could not be verified, but Rudolph Atallah, the former counterterrorism director for Africa in the Office of the Secretary of Defense, said it makes sense.
"Gadhafi bought everything under the sun," said Atallah, a retired U.S. Air Force lieutenant colonel, who has traveled extensively to Mali on defense missions. "His weapons depots were packed with all kinds of stuff, so it's plausible that AQIM now has surface-to-air missiles."
Depending on the model, these missiles can range far enough to bring down planes used by ill-equipped African air forces, although not those used by U.S. and other Western forces, he said. There is significant disagreement in the international community on whether Western countries with their better equipment will carry out the planned bombardments, which could significantly affect the outcome.
Another factor in the success of a military intervention would be the reaction of the people, who, unlike in Afghanistan, have little history of extremism. Malians have long practiced a moderate form of Islam, where women do not wear burqas and few practice the strict form of the religion.
The Islamists' recent advances draw on al-Qaida in the Islamic Maghreb's near decade of experience in Mali's northern desert, where Fowler and his fellow U.N. colleague were held captive for four months in 2008, an experience he recounts in his recent book, "A Season in Hell."
Originally from Algeria, the fighters fled across the border into Mali in 2003, after kidnapping 32 European tourists. Over the next decade, they used the country's vast northern desert to hold French, Spanish, Swiss, German, British, Austrian, Italian and Canadian hostages, raising an estimated $89 million in ransom payments, according to Stratfor, a global intelligence company.
During this time, they also established relationships with local clans, nurturing the ties that now protect them. Several commanders have taken local wives, and Hamaha, whose family is from Kidal, confirmed that Belmoktar is married to his niece.
Fowler described being driven for days by jihadists who knew Mali's featureless terrain by heart, navigating valleys of identical dunes with nothing more than the direction of the sun as their map. He saw them drive up to a thorn tree in the middle of nowhere to find barrels of diesel fuel. Elsewhere, he saw them dig a pit in the sand and bury a bag of boots, marking the spot on a GPS for future use.