Justice for Peace (JEP), which is investigating crimes committed during the conflict between the ex-guerrillas and the government, said in a statement that the letter it received September 30 sought to "tell the truth, clarify the facts and take responsibility" for several killings.

In addition to the murder of Alvaro Gomez, on November 2, 1995, they included the killing of a retired general, Fernando Landazabal, in 1998, and of former peace adviser Jesus Antonio Bejarano in 1999.

This revelation drew significant attention in Colombia, where it had been thought widely that Alvaro Gomez, a Conservative Party leader and three-time presidential candidate, was targeted by political rivals allied with the military and drug traffickers.

The letter is signed by former rebel commanders Julian Gallo, Pastor Alape and Pablo Catatumbo.

Two of them, Gallo and Catatumbo, now sit in the Colombian Congress, as part of the peace accords signed in 2016 that ended a nearly six-decade armed conflict between the former rebels and the government.

The Marxist guerrillas of the Revolutionary Armed Forces of Colombia (FARC), once the most powerful in the Americas, laid down their arms and transformed into a political party.

As part of the 2016 agreement, its main leaders pledged to confess their crimes before the Justice for Peace and to compensate the victims or their families, in exchange for alternative sentences to prison.

If they do not keep their commitment, they will be brought before ordinary justice.