After a homemade bomb killed one police officer and wounded 25 other officers and 4 civilians near Bogotá’s Santamaría Bullring on Sunday, the mayor’s office and the national government have pledged to do “everything in their power” to find the culprits, but as of yet, no motives have been established and no arrests have been made. Bogotá Mayor Enrique Peñalosa has ruled out the involvement of animal rights activists.

Currently, the most-accepted hypothesis is that the culprits were three men between the ages of 20 and 30 who, according to authorities, are part of the National Liberation Army’s (ELN) urban militia. The ELN has not commented on the attack.

“(The bomb) was placed there to do damage to the police. It was activated remotely, probably with a cellphone. With the security cameras in the area, we have established a timeline. We have already identified suspects,” Defense Minister Luis Carlos Villegas told La W Radio. “Does it have anything to do with the pamphlet bombs from a few weeks ago? The most probable hypothesis is that it does, and that it is the ELN.”

The bombing, which was the fourth to hit in Bogotá this year, has heightened tension in the city of a coordinated effort stir fear and target police officers. In recent years, an estimated 30 grenade attacks, homemade bombs, and pamphlet bombs, which are rudimentary explosive devices that discharge pamphlets when they explode, have taken place in Bogotá, according to El Espectador newspaper.

The only arrests made in bombing cases made during that time was a group of students and professors that authorities believed was responsible for attacks on healthcare provider Saludcoop. The students and professors were later released. No other arrests have been made.

In December, two alleged ELN urban militiamen shot to death a police assistant who was patrolling the Torca electrical substation in north Bogotá. The assailants then placed 500 grams of ammonal, an industrial explosive often used in mining and quarrying operations, near the police assistant’s body, according to police. When officers responded to the murder, the men remotely detonated the explosive, injuring six police officers. It is unclear if these two men were also involved in Sunday’s attack.

Juan Camilo Restrepo, the government’s chief negotiator with the ELN, said that any rebel involvement in the attack would “complicate talks.” But given the continued peace talks with the rebel group in Ecuador, it is unclear how much evidence the government has against the ELN, which denied involvement in the December attack.

In January, a pamphlet bomb exploded near a tax office on Carrera Séptima. The pamphlets that scattered into the streets criticized the government’s recent tax reform. A group that calls itself the People’s Revolutionary Movement (MRP) claimed responsibility for the explosion, and allegedly hung a flag from the fifth floor of the apartment building near where the bomb was detonated that read: “Tax reform is hunger for the poor.” Nobody was injured in the explosion.

The occupants of the apartment told police that two men acted as if they were interested in renting an apartment in the building. Once inside, the suspects climbed to the fifth floor and bound and gagged at least one person.

In February, a homemade bomb exploded in Teusaquillo near an Iranian restaurant almost one year after another bomb was placed in the restaurant’s bathroom.

Authorities have said that Sunday’s attack in La Macarena shares some similarities with other attacks, especially the December attack on the Torca electrical substation, and thwarted 2015 bomb attack that was placed near the same spot as Sunday’s attack. The 2015 bomb was discovered by authorities, who then carried out a controlled explosion. No arrests were ever made.

On Sunday, at least one unknown assailant was captured on video placing what authorities believe was an explosive device containing two kilograms of shrapnel next to a cable box near El Pit, a popular hostel with international backpackers located one block from the bullring. The bomb was detonated at 10:32 a.m., just hours before a bullfight as police in riot gear gathered in preparation for scheduled protests against bullfighting.

President Juan Manuel Santos called the bombing “an attack against police who guaranteed safety in La Macarena.”