By Rachel Van Dongen | Correspondent of The Christian Science Monitor

BOGOTÁ, COLOMBIA - The mysterious disappearance of Carlos Castaño, the storied paramilitary chieftain who helped found Colombia's right-wing death squads in the 1980s, has thrown the country's already rocky peace process into turmoil.

President Alvaro Uribe had been in negotiations with Mr. Castaño, trying to demobilize some 20,000 paramilitary combatants. Now analysts and paramilitary leaders both say that the government and the United Self-Defense Forces of Colombia (AUC), the group originally organized by Castaño, are likely to take a harder line in negotiations that were already threatening to fail, possibly leading to renewed violence.

Although Castaño's whereabouts are still unknown, one of his bodyguards told authorities that a group of 50 AUC assassins arrived at Castaño's farm at 2 p.m. on April 16 and killed at least two of his bodyguards. Rodrigo Franco, a dissident paramilitary commander, says that Castaño is dead, according to an e-mail sent to the Monitor.

Mr. Franco says that three AUC leaders - Salvatore Mancuso, Diego Fernando Murillo, and Castaño's brother, Vicente - were responsible for the attack. All three are wanted by the US for drug trafficking and are known to be associated with the wing of the AUC that is heavily involved in the drug trade.

On Tuesday, in a sign of a tougher approach to negotiations, Mr. Uribe issued his harshest statement to date about the foundering peace process. He said that the paramilitaries must respect their oft-violated cease-fire and called on them to concentrate in specific zones where compliance with the cease-fire could be monitored.

"They must move toward demobilization. If not, the government will continue fighting them until they are destroyed," he said.

Furthermore, the president reiterated the government's support for its newly proposed "alternative penalty" project that would give reduced sentences to demobilized paramilitary killers and criminals. Under the new proposal, which Uribe termed "generous," those guilty of certain crimes would be incarcerated for five to 10 years. And he repeated that waiving extradition to the United States was not an option. Castaño, Mr. Mancuso, and Mr. Murillo have all been indicted in the US on charges of drug trafficking. The AUC continues to say that incarceration and extradition are dealbreakers.

Defense analyst Alfredo Rangel says that following Castaño's disappearance, the government and the AUC will now find it harder to reach an accord.

Mr. Rangel says that Castaño appeared disposed to accept government terms, which probably provoked the attempt on his life. "I think they [other paramilitary leaders] saw Castaño as too soft," he says.

In recent months, Castaño had spoken out against the influence of narcotraffickers within the AUC. After Castaño's disappearance, Mancuso suggested that he had negotiated his own surrender to US authorities in order to cut a deal. The US Ambassador to Colombia, William Wood, strongly rejected that suggestion.

Mr. Franco predicts that the peace process will collapse "sooner or later" but would continue in the near term.

"The government needs the process to show some result of pacification without violence. Also, for Uribe's re-election," Rodrigo says, referring to an attempt by Uribe to change the Constitution to allow him to seek a second term.

The paramilitaries were founded by Castaño and his brother, Fidel, in the 1980s as a loose association of fighters financed by wealthy landowners to stop the advances of the leftist Revolutionary Armed Forces of Colombia. In 1997, Castaño united the group under one umbrella organization, the AUC.

The group later broke into several regional factions controlled by strongmen with connections to narcotrafficking. By some estimates, the AUC controlled 40 percent of Colombia's cocaine industry. Castaño provoked rifts in the organization by denouncing such links, despite his own indictment on drug-trafficking charges.

This is not the first time a Castaño brother has mysteriously disappeared. A leftist militia killed Fidel Castaño in 1994, but Carlos Castaño covered up the death until the 2001 publication of his bestselling book, "My Confession."

"Carlos Castaño knew that mystery turns warriors into myths that are fed by the wonder of the men," wrote Mauricio Aranguren Molina, who co-wrote Castaño's book. "That is, during unpredictable times, it prolongs their lives after death."