Rebels of the Revolutionary Armed Forces of Colombia (FARC) have denied planting a massive car bomb which killed 36 people in the capital Bogota last month.
The attack was very sophisticated, officials said
In an announcement on its internet site, the FARC - Colombia's largest rebel group - blamed the attack on what it called "state terrorism".
However, the Colombian Government maintains it has enough evidence to prove that the FARC was responsible for the blast at an exclusive social club.
The BBC's Jeremy McDermott in Colombia says the guerrillas do not have a history of denial and when they have issued such denials in the past they have proved correct.
Well over 100 people were injured in the blast at Club El Nogal on 7 February - the worst attack in a Colombian city for a decade.
According to local media reports, a children's party and a wedding reception were being held in the multi-storey building at the time.
It is believed a car packed with explosives was left in a garage on the third floor of the club - a place frequented by Colombia's political elite and foreign diplomats.
Vice President Francisco Santos has rejected the rebel statement, saying there was not the slightest doubt the attack was carried out by the FARC.
The guerrillas have been waging war against the government for 38 years.
Our correspondent says that if the FARC was not behind the El Nogal bomb, the list of possible suspects is long and undistinguished, including another guerrilla group, right-wing paramilitaries, drug lords, or disaffected elements within the military.
President Alvaro Uribe has declared a state of emergency to counter the war, which has claimed many thousands of lives.