Colombian President Alvaro Uribe has accused the Farc rebel group of murdering 11 politicians they were holding captive last week.
Only one of the 12 hostages is said to have survived the killings
The rebels have said the hostages died in crossfire when one of their camps, in a western region, was attacked.
But Mr Uribe said there had been no military operations in Valle del Cauca region where the hostages were held.
The 11 provincial deputies were from a group of more than 50 hostages he wants exchanged for rebel prisoners.
Mr Uribe began releasing jailed rebels at the beginning of the month in what he described as a goodwill gesture, but the Farc said the move was a farce.
'Crime against humanity'
The Farc said the hostages died when unidentified gunmen attacked the jungle prison camp where they were being held on 18 June.
A 12th member of the group survived because he was elsewhere at the time of the attack, it added.
But speaking in a live, televised address, Mr Uribe described the Farc statement as a ploy to cover up a "crime against humanity".
"No rescue mission was under way," he said. "They were deliberately assassinated."