A commander of Colombia's right-wing paramilitary AUC has alleged that two members of the current government previously conspired with the group.
Mancuso gave his testimony as part of a peace deal
In a court hearing in Medellin, Salvatore Mancuso said he met the country's current vice-president and defence minister in the 1990s.
His claims have yet to be verified by the attorney-general's office.
The defence minister has dismissed them. The government denies any co-operation with the AUC.
But the revelations are explosive and potentially crippling for the government, says the BBC's Jeremy McDermott in Medellin.
Mancuso, a high-ranking member of the United Self-Defence Forces of Colombia (AUC), gave evidence as part of the 2003 peace deal which has led to the demobilisation of thousands of paramilitary fighters.
He has alleged that Colombia's Defence Minister Juan Manuel Santos met paramilitaries between 1996 and 1997 to help plot the overthrow of then President Ernesto Samper.
Mancuso also claimed he met Vice-President Francisco Santos, the defence minister's cousin, in the late 1990s to discuss the creation of a new militia group to fight guerrillas in the capital, Bogota.
At the time, Francisco Santos was an editor for Colombian newspaper El Tiempo.
Mancuso also gave dates, places and the names of others who attended the meetings.
So far around a dozen congressman have been jailed on charges that they collaborated with the paramilitaries.
But Colombia's government has insisted the paramilitaries were created independently of the state, and that only a "few bad apples" in the security forces are guilty of cooperating with them.
The AUC has been accused of drug smuggling and was involved in a long-running conflict with state forces and left-wing rebels, but is now engaged in a peace process.
Tens of thousands of people have been killed in four decades of fighting and many more have been displaced.
In 2003, Mancuso was sentenced in absentia to 40 years in prison for his part in a 1997 massacre.
Under the peace deal reached with the government, paramilitary fighters are eligible for reduced jail terms - of no more than eight years - if they give details of their involvement in torture, killings and other crimes.
The peace deal has been criticised by human rights activists who say it amounts to an amnesty.