The 11 politicians who died while being held by Farc rebels were killed during an accidental clash between factions, Colombia's intelligence chief has said.
Only one of the 12 hostages survived the killings
Andres Penate said intercepted communications showed the left-wing movement had shot dead the hostages after coming across another rebel unit.
Thinking they were security forces, commanders ordered the hostages to be killed rather than let them be rescued.
The Farc said in a statement that they were investigating the incident.
The group had previously insisted the politicians were killed in crossfire when an "unidentified military group" attacked their jungle camp in the western Valle del Cauca region on 18 June.
But the head of the DAS intelligence agency denied there had been any military operations in the area, saying informers and communication intercepts proved the Farc were responsible for the deaths.
Mr Penate said evidence showed the commander of the 60th Front, who is known as El Grillo, or the Cricket, ordered the politicians to be shot dead after his fighters clashed with the 29th Front.
Mr Penate then presented the transcript of a radio conversation, in which an unidentified Farc rebel reportedly asks about the sole hostage who was not killed in the incident.
The rebels then "tampered with the scene of the crime" and attempted to move the bodies before they are handed over, Mr Penate said.
"They're trying to deceive the public, pretending that the hostages were located in the proposed safe heaven so that they can blame the government for their deaths," he told reporters on Saturday.
"Fools! Assassins! Liars! Now they want to consummate the lie," he added.
The BBC's Jeremy McDermott in Bogota says that, if it is true, the massacre will pile yet more pressure on the Farc, which after more than 40 years of fighting, has reached its lowest level of public support.
The Farc, or Revolutionary Armed Forces of Colombia, is attempting to overthrow the government and impose a socialist state.
The politicians were among a group of some 60 hostages - including Colombia's former presidential candidate Ingrid Betancourt and three US defence contractors - who the Farc want to swap for hundreds of their imprisoned comrades.