If confirmed, the death of Manuel Marulanda, who organized the Revolutionary Armed Forces of Colombia guerrillas in the 1960s, would be the heaviest blow yet to Latin America's oldest insurgency, already weakened by a military setbacks.
Reports that Marulanda, also known as "Sureshot," had died or fallen ill have surfaced before, but they were never confirmed. He was in his late 70s and has not been seen in public since failed peace talks more than five years ago.
"Through military intelligence, we learned Pedro Antonio Marin, alias Manuel Marulanda or Sureshot, the principal chief of the FARC, is dead," the Defense Ministry said in a statement. "The cause of death is still to be confirmed."
The legendary rebel chief either died of a heart attack, according to FARC information, or during military bombardments in late March in the southern jungles where he spent much of guerrilla life, the ministry said without providing proof.
President Alvaro Uribe, speaking to reporters, stopped short of claiming victory against the FARC. "These sources are serious, we hope," he said.
Uribe's father was killed during a botched FARC kidnapping two decades ago and is popular for cracking down on the rebels.
A shy, peasant who once sold candy for a living, Marulanda took up arms in a left-wing insurgency fighting for social justice in the 196Os. But after four decades, the FARC has been weakened by Uribe's U.S.-backed security campaign.
With little popular support, Marulanda's rebels have been driven into remote jungles and mountains, but remain a potent force in some areas, bolstered by funds from drug smuggling. U.S. and E.U. officials list the FARC as terrorists.
FARC: A DYING GIANT
Violence from the conflict has eased as Uribe, backed by billions in U.S. military aid, has sent troops to retake areas once under guerrilla control. Often using homemade landmines and mortars, the FARC is still battling security forces.
But several top FARC commanders have been killed or captured recently as the rebels struggle against increasing military pressure and growing desertions from their ranks.
Experts said Marulanda's authority was always a cohesive element in the ranks of the FARC, which during its peak had 17,000 fighters but is now closer to 9,000 combatants.
"The FARC is like a dying giant, dying slowly, but this is the beginning of the end," Pablo Casas, an analyst at Bogota think tank Security and Democracy. "I don't see any factor they can use to keep a strong structure. It will start collapsing."
The FARC's No. 2, Raul Reyes, was killed in March when Colombian troops raided his base inside Ecuador, in an operation that triggered a regional crisis with Venezuela and Ecuador briefly ordering troops to Colombia's frontier.
Colombian and U.S. officials say files found on Reyes' computer indicate Venezuela's President Hugh Chavez and Ecuadorean leader Rafael Correa provided financial support or backing to the rebels. Both leaders deny those charges.
While Marulanda's death may be a blow to the FARC's structure, questions linger over the fate of scores of hostages the guerrillas have held for years, including French-Colombian politician Ingrid Betancourt and three Americans.
Uribe said the government had been contacted by rebels who were willing to surrender and hand over hostages they are holding including Betancourt. But he said another group of FARC commanders were determined to fight on.
Attempts to broker a deal to free kidnap victims are deadlocked over rebel demands that Uribe demilitarize a rural zone for negotiations. He refuses, saying that would allow the FARC to rearm and regroup in a strategic region.