A confused clash between two bands of rebels led a commander to order the death of 11 hostage lawmakers last month, the head of Colombia's intelligence agency said.
Andres Penate, the head of the DAS intelligence agency, told a Bogota press conference on Saturday that rebels from the Revolutionary Armed Forces of Colombia (FARC) 60th Front killed the hostages when they mistook another group of insurgents for army troops.
His account contradicted that of FARC which announced late last month that the legislators had died in the crossfire when an "unidentified military group" attacked the camp where they were being held.
There was no immediate response from the FARC and no way to reach its officials.
Penate said testimony from deserting guerrillas and intercepted radio communications showed that the 60th Front commander, known as "El Grillo," ordered the lawmakers killed during the clash with the 29th Front between June 10 and June 16 in the state of Narino.
Penate presented a transcript of a radio conversation in which an unidentified guerrilla apparently asks about the sole hostage lawmaker who was not killed in the incident.
President Alvaro Uribe has accused the rebels of murdering the captives in cold blood and challenged the FARC to give the bodies to an international commission led by the Red Cross so that forensic experts can determine how they died.
The FARC has issued statements from the jungle offering to hand over the bodies, but it has not yet done so.
"It's saddening there are so many rumors floating around," said Fabiola Perdomo, the widow of a slain congressman. "All we want is the truth."
Penate said the same radio communications indicate that the FARC is trying to move the bodies before a handover to make it seem that they were in an area the rebels demand be made a demilitarized zone for talks with the government over a swap for another 50 hostages, including three US defense contractors.
"They're trying to deceive the public, pretending that the hostages were located in the proposed safe haven so they can blame the government for their deaths," Penate said.
"These FARC bandits know that the longer they delay handing over the cadavers, the more difficult it will be for forensic experts to determine how they were killed," Uribe said on Saturday. "Fools! Assassins! Liars! Now they want to consummate the lie."
The 12 lawmakers were kidnapped in April 2002 in the southwest city of Cali, in a daylight raid on the state legislature by rebels dressed as soldiers.