In the wake of last Thursday's double bombing in the Colombian capital, the government is blaming the rebel group National Liberation Army.
Fifteen people were arrested following police raids on Wednesday, mostly students in their 20s and 30s. The arrests follow bombings carried out on July 2 against two Bogota pension offices. The explosives left no fatalities but injured ten.
Immediately following the attacks, President Santos said in a televised statement that all the evidence "points in the direction [of the ELN]." But so far, conclusive evidence hasn't been made public.
Authorities told the news magazine Semana a man who was identified as a professor at Universidad Nacional was the leader of a terror cell belonging to the rebel army known as ELN.
A coalition of human-rights groups identified those arrested as mostly philosophy, pedagogy, and literature students involved in rights organizations. The People's Congress demanded the liberation of those arrested.
The older suspect, David Camilo Rodriguez Hernandez, reportedly told authorities he was unemployed, and the university said it had no relation to the man.
The ELN is Colombia's second largest rebel group. The guerrilla has been in exploratory peace talks to end its conflict with the government, though nothing has been formalized.
Colombia is in its third year of delicate peace talks with the the larger guerilla, Revolutionary Armed Forces of Colombia, or FARC, to end the 51 years of war. On Friday, two Colombian soldiers were killed in gunfire with FARC in the Caquetá department, reports said.
'Absolutely all our structures and fronts do not know any of those captured and not one of them is an ELN militant.'
The rebel group told VICE News it is unfamiliar with all the detainees. Two students were released due to lack of evidence, though 13 remained under arrest on Friday, according to local press reports.
"Absolutely all our structures and fronts do not know any of those captured and not one of them is an ELN militant," an unnamed representative from Radio Patria Libre, the group's official radio channel, said in an interview via Twitter.
The bombings raised tensions in Bogota, a city that suffered during a wave of fatal bombing attacks in the height of the conflict in Colombia involving drug traffickers in the 1980s and 1990s.
Earlier this year, a string of similar unexplained bombings rattled the city. Authorities also said then that the ELN might have been involved, but the string of four low-impact attacks on targets such as political offices was never solved.
A similar such wave of unexplained explosives in recent years has affected cities in Chile.
This week, Amnesty International accused Colombian authorities of violating the detainees' presumption of innocence by calling the suspects terrorists without presenting evidence in statements to the press.
Adam Isacson, a senior associate at the rights watchdog Washington Office on Latin America, said several other theories have not been discounted on the attacks.
"It could be the ELN, trying to show that they have the capacity to do damage in Bogota," he told VICE News. "Or it could be right-wing saboteurs trying to pin the attacks on the FARC, which if successful, would do great damage to the peace talks."
He added: "Or it could simply be organized crime sending a message to an extortion target who hasn't paid up."
For updates on the conflict in Colombia, follow Joe Parkin Daniels on Twitter: @joeparkdan