Colombia's chief prosecutor's office has opened a preliminary criminal investigation into former President Alvaro Uribe over allegations he sponsored a killer far-right militia as a regional governor in the 1990s.

The probe ordered on Jan. 2 was made public Tuesday by Uribe's attorney, Jaime Granados. Uribe denounced it on Twitter as defamation based on the testimony of "manipulated prisoners."

It stems from 2011 accusations by leftist congressman Ivan Cepeda that a far-right militia that killed suspected rebel sympathizers operated from a ranch owned by Uribe and his brother Santiago.

The prosecutor in charge of the case is Martha Lucia Zamora and she says it is based on the testimony of former militia members. Granada says it is built on lies.

Uribe was Colombia's president from 2002-2010.