IBAGUE, Colombia — Somewhere between canyon and crag, between rushing rivers and clouds that hide the sky, between 12,000 feet of mountains, Colombia's most-wanted rebel is believed to be hiding.

A special military force 5,000-strong, supported by thousands more, is tasked with finding the leader of Colombia’s largest rebel group, the Revolutionary Armed Forces of Colombia, or FARC.

Guillermo Leon Saenz, who goes by Alfonso Cano, has long been an elusive target for the military. So was the FARC’s No. 2 commander, known as “Mono Jojoy,” until he was killed last month in an air raid over his jungle hide-out.

Jojoy was thought of as untouchable. His death has raised expectations for what many hope will be the next military victory: Cano’s fall.

“Now is the moment,” said Col. Julio Prieto, commander of the Sixth Brigade in the central department of Tolima. Though the FARC quickly announced a replacement for Jojoy, analysts and military officials expect his death to trigger a degree of disarray and restructuring. The military hopes to exploit that to further debilitate the rebel group.

The challenges, however, are daunting. “You can not compare looking for Cano to looking for Mono Jojoy just because of the terrain itself,” Prieto said.

Cano is believed to be hiding in the Canon de las Hermosas, where the wild terrain poses myriad challenges for the military. Bringing in food and supplies can be complicated. Helicopters often can’t fly because of fog that never seems to lift. The huge changes in altitude impose great physical demands on troops, and up high, some of them succumb to hypothermia. The greatest hazards of all, however, are the countless improvised explosives the FARC has planted in the mountainsides.

These have long been the stomping grounds of the FARC’s Central Bloc, whose fronts extend across Tolima and neighboring departments.

To chip away at Cano’s ability to wage war while in hiding, army troops are going after the various fronts of the Central Bloc. “We are making many consistent efforts to take down the structures that, one way or another, are offering him security,” said Prieto, who has 12,000 troops covering Tolima.

In the last year, seven of the Central Bloc’s 12 leaders have been killed. The loss of guerrillas to air bombings, capture or desertion further weakens the rebels. Two weeks ago, the last seven members of the 50th front, 110-members strong in 2003, turned themselves in to join a government program that provides benefits to demobilizing guerrillas. Many of them provide information — such as the location of guerrilla camps and caches of explosives — that have become key to military efforts.

Despite the signs of military encroachment, Cano’s ability to shield himself may keep the army at bay for a while yet. “I don’t see the fall of Alfonso Cano coming anytime soon,” said Leon Valencia, director of the Nuevo Arco Iris think-tank that studies the conflict.

Cano, a former university professor who wears spectacles and a beard, keeps his security ring small. Felipe Salazar, a former regional leader for the FARC who worked closely with Cano, estimates the top commander has no more than 25 of his most dedicated and loyal associates forming his immediate circle of protection.

That high degree of loyalty may make it less likely one of them turns on their leader and provides intelligence to authorities (ex-guerrillas provided crucial information to the military for the attack on Mono Jojoy’s camp for a large cash reward).

The guerrilla units that operate in Cano’s extended periphery tend to be small, disperse, mobile and less conspicuous than the concentric rings of estimated 400 guerrillas surrounding Mono Jojoy’s camp when it was bombed.

Information about Cano’s location also appears to be held very closely, and unknown to the rank-and-file who continue to desert.

“I didn’t know even know where he was, because his people are very distrusting, and I had the confidence of the commanders of every front,” said an ex-guerrilla who held a position of great trust as a human courier running messages and USB keys between various front commanders across Tolima and neighboring departments. The 29-year-old, who cannot be named for security reasons, escaped two weeks ago after spending 20 years in the FARC.

“There could be 30,000 or 40,000 military looking for him and it wouldn’t necessarily make a difference,” said Salazar. “They don’t realize who they’re looking for.”

Prieto is more optimistic. “Every day, [Cano] is more closed in,” he said.

Capturing or killed Cano would undoubtedly deal a major loss to the insurgency he leads. He brings both military leadership as well as political brains, and is trying to bring the rebel group back to its ideological roots.

But cutting off a snake’s head won’t necessarily kill the snake. Changes in structure and modus operandi that Cano has been leading will likely continue without him, Salazar said. The rebels have seen their numbers drop from about 18,000 in 2001 to an estimated 8,000. They remain Latin America’s longest-running insurgency.

Cano appears to be fully aware of the need for the FARC to adapt in order to stay alive.

Their reduced ability to launch military attacks means the FARC have increased their use of improvised explosives. Their units operate more independently and they have reverted to sometimes using human messengers to avoid being susceptible to intercepted communications. “The strength of bamboo is in its flexibility,” Salazar remembers Cano saying.