Colombia's attorney general's office has reopened a preliminary criminal investigation against former President Alvaro Uribe.
Mr Uribe is accused of involvement with far-right paramilitary groups while he was a state governor in the 1990s.
The case is based on testimonies by imprisoned former right-wing militia members.
Mr Uribe's lawyers deny all the charges and said they are "full of suppositions, imprecisions and lies".
On Twitter, the former president said the allegations were "criminal vengeance" by imprisoned drug-dealing paramilitaries.
Mr Uribe held office between 2002 and 2010 and waged a war against left-wing rebels from the Farc (Colombia's Revolutionary Armed Forces).
After a peace deal in 2003, tens of thousands of paramilitaries were demobilised and surrendered in exchange for reduced jail terms and protection from extradition.
The latest accusations against Mr Uribe were presented by Pablo Hernan Sierra, also known as Alberto Guerrero, and Juan Guillermo Monsalve.
They had already been dismissed by the courts, but last week the attorney general's office decided to reopen the case.
The prisoners say Mr Uribe had strong links to the United Self-Defence Forces of Colombia (AUC) and was involved in ordering a killing, while governing the state of Antioquia.
The former head of state promised to present his reaction to the media on Wednesday.
A number of closes aides of Mr Uribe's, including his cousin Mario Uribe, have been found guilty of collaborating with the AUC.
He, however, has always maintained he has had "zero paramilitaries" in his life.