BOGOTA, Feb 27 (Reuters) - The National Liberation Army, Colombia's second-biggest guerrilla group, admitted on Thursday it was behind an attack last weekend on a leftist presidential candidate but said it was returning fire after being shot at first.
A convoy carrying Aida Avella, 65, a contender from the Patriotic Union party, came under fire on Sunday as it traveled on a dirt road in the cattle-ranching and oil-rich northeastern province of Arauca, where rebels of the ELN have a strong presence.
The ELN, in a statement, said it was attempting to stop the caravan of 12 vehicles with darkened windows to ascertain who was inside but came under fire first.
"Instead of stopping, from the interior of one of the vehicles our guerrillas were fired at, and to protect themselves they fired back," the ELN statement said.
"We deeply regret the incident and apologize to the candidate Aida Avella. At the same time we will take the necessary precautions to ensure that this does not happen again," it said.
The government earlier this week blamed the ELN and said police and military intelligence had overheard conversations between rebel members referring to the shots fired.
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 as little as 1 percent of support among the electorate for the May 25 elections, according to a February opinion poll that put President Juan Manuel Santos in the lead with 34.7 percent.
Earlier this month she denounced death threats she said had been made against her.
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.
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 it has steadily gained power since, counting as many as 5,000 fighters, financed by "war taxes" levied on landowners and oil companies. (Reporting by Helen Murphy; Editing by Dan Grebler)