Factbox: Colombia's left-wing guerrillas the ELN

BOGOTA - Colombia's second-largest rebel group, the National Liberation Army or ELN, have begun exploratory peace talks with the government, days before a tight reelection battle for President Juan Manuel Santos, the presidency said in a statement on Tuesday. If talks are launched, the ELN will follow in the footsteps of larger rebel group the Revolutionary Armed Forces of Colombia, or FARC, who have been holding negotiations with the government for 19 months. Here are key facts on the ELN: * The guerrilla group of about 3,000 fighters, considered a terrorist organization by the U.S. and EU, was founded in 1964 by radical Catholic priests inspired by the Cuban revolution. * The ELN, like Colombia's larger and more famous rebel group the FARC, has been weakened by a 10-year U.S.-backed military offensive, which has nearly halved the ELN's number of combatants from an estimated 5,000 in the mid-1990s. * The group operates largely in Colombia's northeast and frequently attacks oil infrastructure such as pipelines, and has kidnapped energy sector workers. * The ELN has sought peace before, holding talks in Cuba and Venezuela between 2002 and 2007. Experts say there was a lack of will on both sides to agree on a final peace plan. * Though government sources now believe the ELN's primary source of income is drug trafficking, ransom and protection payments made up much of the rebel movement's revenue until the mid-2000s. (Reporting by Julia Symmes Cobb; Editing by Cynthia Osterman)