BOGOTA, Colombia—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 and made public Tuesday by Uribe's attorney, was denounced by Uribe on Twitter as "the defamation of imprisoned criminals" he claimed had been "manipulated."

The case stems from 2011 accusations by leftist congressman Ivan Cepeda that a far-right militia that killed suspected rebel sympathizers was formed on a ranch in the Antioquia state town of San Roque owned by Uribe and his brother Santiago.

Cepeda alleges the militia was created while Uribe was governor of Antioquia from 1995 to 1997, a period during which Uribe also promoted the spread of citizen's self-defense groups called Convivir that human rights groups accused of abuses including extra-judicial killings.

As Colombia's president from 2002-2010, Uribe engineered the demobilization of thousands of members of far-right militias. The militias were first created in the 1980s by ranchers and drug traffickers to combat kidnapping and extortion by leftist rebels but evolved into lawless criminal gangs that prosecutors say killed several tens of thousands.

Cepeda called the opening of the investigation against Uribe "without doubt the most important step by Colombia's judicial system in the terrible case of parapolitics."

The parapolitics scandal, which broke open in late 2006 in large part due to the efforts of Bogota's current mayor, Gustavo Petro, exposed symbiotic ties between many right-wing politicians and the illegal far-right militias known as paramilitaries.

Uribe's leftist critics have long sought a smoking gun that proves he sponsored the militias. Uribe says they haven't found one because none exists.

The prosecutor in charge of the newly opened case, Martha Lucia Zamora, told The Associated Press that it is based on the testimony of jailed former militia members that Cepeda initially obtained.

The two key witnesses are Pablo Hernan Sierra and Juan Guillermo Monsalve.

Cepeda has said Sierra has admitted to forming a far-right militia that operated from the Uribe ranch, known as Guacharacas. And he says Monsalve is the son of the former manager of the Uribe ranch and claims Alvaro Uribe didn't just support the right-wing militias but "presumably ordered massacres" in San Roque.

Jaime Granados, the Uribe attorney, says both men's testimony against his client is "plagued by supposition, imprecision and lies."

The investigation, which is almost certain to lead to Uribe being summoned for an interrogation, is not the first against Uribe. Several have been initiated in Congress, the only body empowered to judge Uribe for his actions as president, but are stalled in its accusations commission.

The most important is related to domestic spying by the now-dissolved DAS security agency, which illegally eavesdropped on Supreme Court justices, journalists, politicians and opposition activists.

Uribe says he's innocent of the accusations and calls the attempts to prosecute him political reprisals.

His government made major battlefield gains against Colombia's main leftist rebel force with U.S. military backing, laying the ground in the eyes of many Colombians for peace talks that opened in Cuba in November and resume on Monday following a holiday break.

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Associated Press Writer Frank Bajak contributed from Lima, Peru.