Colombia's ELN rebels likely behind Bogota blasts: president

The bombs were detonated in front of two offices of the Porvenir pension fund, a subsidiary of the Grupo Aval financial conglomerate, injuring seven people. The government pledged to provide a heavier army and police presence in Bogota.

"The information we have indicates that the ELN were responsible," Santos said in a televised address on Friday, adding that the group was behind a handful of other attacks or foiled attacks in the city over the last year.

Those responsible would be brought to justice, he said.

The motive for the attack may have been the anniversary of the ELN's founding 51 years ago, Santos said, and this was one theory discussed at a security meeting with government officials and senior members of the armed forces on Friday.

Grupo Aval is owned by Colombia's richest man, Luis Carlos Sarmiento.

The ELN, with an estimated 2,000 members, has held preliminary talks with the government on the possibility of starting peace negotiations but it has continued to attack army troops and infrastructure.

The attacks come at a time of heightened security concerns in the Andean country after the ELN's larger counterpart, the Revolutionary Armed Forces of Colombia or FARC, unleashed a wave of bomb attacks against oil pipelines in recent weeks.

Those attacks caused thousands of gallons of crude oil to spill into major rivers, causing an environmental disaster whose effects oil companies say will be felt for two decades.

The FARC and ELN have been fighting successive governments for five decades since forming to demand land reform and more equitable wealth distribution, in a conflict that has killed more than 220,000 people and created one of the world's largest internally displaced populations.

Though the ELN has yet to agree to full negotiations, the FARC has been holding peace talks with the government for two and a half years in Cuba, and the discussions have continued despite renewed violence since the group called off a five-month ceasefire.

(Reporting by Peter Murphy; Editing by Eric Walsh)