MEXICO CITY, Oct 9 (Reuters) - Heriberto Lazcano, the slain boss of the Zetas drug cartel, was once an elite special forces soldier before switching sides to join the criminals he was charged to fight, eventually becoming one of Mexico's most feared and brutal kingpins. Known as "The Executioner" and "Z-3," Lazcano was killed on Sunday in a gun battle with Marines in northern Coahuila state, Mexico's Navy said. If confirmed, it would be the biggest coup yet for President Felipe Calderon in his war on drug cartels. However in a bizarre twist, Lazcano's body was snatched by armed men from a funeral home just hours later, shrouding the incident in mystery. While Calderon said "all available evidence" indicated the cartel chief had been killed, including finger prints, he did not say he knew for sure that he was dead. Lazcano was mistakenly reported killed in 2007 after a clash with the military. Lazcano was one of Mexico's most wanted men and U.S. authorities offered a reward of up to $5 million for his capture. Only Joaquin "Shorty" Guzman, boss of the Sinaloa Cartel, would represent a bigger prize to the government. Born into rural poverty, Lazcano deserted from a Mexican army unit formed to combat the drug gangs in 1998 and joined the Gulf cartel's vicious enforcement wing, the Zetas, where he quickly won power thanks to his merciless slaying of rivals. The Attorney General's office has said Lazcano was believed to own a ranch with a pit containing lions and tigers, into which he used to hurl his victims. The Zetas, named after a military call sign, split from the Gulf Cartel in 2010, and have continued to expand even as rival cartels joined forces against them. Under Lazcano's leadership, the Zetas grew into a feared organization of more than 10,000 gunmen with operations stretching from the Rio Grande, on the border with Texas, to deep into Central America. Armed with a huge arsenal of automatic weapons, dynamite, grenades and even rocket launchers, the Zetas have waged a gruesome battle for supremacy with a coalition of rival drug gangs from Mexico's Pacific state of Sinaloa since 2004. The gang's expansion has pushed out Mexico's older cartels in many areas, giving them a dominant position in the multi-billion-dollar cross-border drug trade, as well as extortion, kidnapping and other criminal businesses. They were blamed for the brutal massacre of 72 foreign migrant workers headed to the United States and the burning of a casino in the affluent city of Monterrey that claimed 52 lives. Hundreds of bodies found in mass graves may have been their kidnapping victims. Rivals, snatched from safe houses and off the streets, were tortured and mutilated by the Zetas, who are believed to have pioneered decapitating gangland rivals, now a grim hallmark of Mexican organized crime. In May, Zetas were blamed with killing 49 people and dumping their headless and limbless bodies on a highway near Monterrey. Lazcano's recruitment drive extended to former elite Guatemalan soldiers known as Kaibiles, who committed human rights atrocities during that country's long civil war, Mexican officials say. But little else was known about the kingpin, who turned his back on opulent displays of wealth and power common among other Mexican drug lords, and kept a low profile. "He is the most secretive of the bosses because he's trained in intelligence," George Grayson, a U.S.-based Mexico expert at The College of William and Mary said of Lazcano at the height of his power. "He's not out there throwing birthday parties or getting musicians to compose songs for him, he's out there to make money," he said, referring to the more flamboyant habits of other drug traffickers. Under Lazcano's command, the Zetas were organized in a cellular structure and low-ranking members know little about overall operations. ARMY DESERTERS TARGETED The group became a key target of Calderon, who made crushing the Gulf cartel and its former enforcers one of his main goals in a military-led offensive involving tens of thousands of troops launched after he took office in 2006. About 60,000 people have been killed in drugs violence since then. Despite the government assault, Lazcano appeared undaunted, openly advertising for soldiers to desert and join the Zetas. The group strung banners from bridges over main roads in the towns of Reynosa and Nuevo Laredo on the U.S. border offering attractive pay to recruit other deserters like Lazcano, who receive measly salaries in the army. In the working class neighborhood in Pachuca, in central Mexico, where Lazcano grew up, he built a vast Roman Catholic brick chapel in 2009. Fronted by a towering cross in light steel, a plaque says openly, "Donated by Heriberto Lazcano. Lord, hear my prayer, attend my petitions, you that are faithful and just." In anticipation of his own death, the kingpin had also built a brick mausoleum nearby, police said. In recent months, the Zetas appeared to be rupturing, with a longstanding rivalry between Lazcano and his deputy Miguel Trevino, alias "Z-40," exploding into violence. Analysts said Lazcano's death could trigger further blood letting as cartel lieutenants battle to fill a power vacuum within his faction of the cartel. "As they don't have a strong leader ... second or third tier leaders could take over the organization. ... It could lead to greater violence," said Vicente Sanchez, a researcher with the Colegio de la Frontera Norte.