By Helen Murphy and Luis Jaime Acosta
BOGOTA, Feb 24 (Reuters) - The National Liberation Army (ELN), Colombia's second-biggest guerrilla group, was behind the attack this weekend on a leftist presidential candidate, the country's top defense official said on Monday.
A convoy carrying Aida Avella, 65, a contender from the Patriotic Union party, came under fire on Sunday as it traveled on a highway in the oil-rich northeastern province of Arauco, where rebels of the ELN have a strong presence.
Police and military intelligence overheard conversations between members of the ELN, referring to the shots fired at her vehicle, Defense Minister Juan Carlos Pinzon told reporters.
"In a specific manner, they talk about how the 12-vehicle convoy mobilized, how they tried to stop the presidential candidate's convoy and when they could not, they opened fire," Pinzon said.
Avella represents UP, a party founded in 1985 after a failed peace process brokered by then President Belisario Betancur with the Revolutionary Armed Forces of Colombia, or FARC, a Marxist rebel group.
Some 5,000 members and supporters of the UP were killed in the years after its creation by right-wing paramilitary groups set up by vigilantes protecting wealthy landowners.
Avella has just 2 percent of support among the electorate for the May 25 elections, according to a February opinion poll which put Santos in the lead with 34.7 percent.
Earlier this month she denounced death threats she said had been made against her.
Her government-supplied armored car was not hit, but another vehicle in the convoy had 14 bullet holes - many of them from her own security detail's weapons, officials said.
Avella had lived in exile for years after surviving an attack in 1996 by paramilitaries who fired a rocket-propelled grenade at the vehicle she was traveling in on a highway crossing the capital, Bogota.
The presidential election will be held amid a complicated backdrop of peace talks with the FARC.
Though many Colombians back the process, it has upset some voters and enraged right-wing politicians who charge that Santos is giving too much away to militants.
The ELN is also seeking a place at the negotiating table. The government wants to bring an end to five decades of conflict that has killed more than 200,000 between the various insurgent groups, civilians and armed forces.
The ELN has battled a dozen governments since it was founded in 1964 and is considered a terrorist group by the United States and the European Union.
Inspired by the Cuban revolution and established by radical Catholic priests, the ELN was close to disappearing in the 1970s. But has steadily gained power since, counting as many as 5,000 fighters, financed by "war taxes" levied on landowners and oil companies, by 2002.
The ELN guerrillas are now believed to number about 3,000. It has sought peace before, holding talks in Cuba and Venezuela between 2002-07, but experts say there was a lack of will on both sides to agree to a final peace plan. (Reporting by Helen Murphy)