A car bomb attack on a police cadet training academy killed 10 people and wounded 65 on Thursday in Bogota, making it the worst such incident in the Colombian capital in 16 years.
The defense ministry said the "terrorist act" was carried out using a vehicle packed with 80 kilograms (around 175 pounds) of explosives.
"All Colombians reject terrorism and we're united in fighting it," President Ivan Duque tweeted.
Vowing to bring those responsible to justice, Duque added: "COLOMBIA is sad but will not bow to violence."
The bomber -- believed to have been killed in the attack -- struck at the General Francisco de Paula Santander Officer's School in the south of Bogota during a promotion ceremony for cadets.
No group has claimed responsibility, but public prosecutor Nestor Humberto Martinez named suspect Jose Aldemar Rojas Rodriguez as the "material author of this abominable crime."
Martinez said Rojas Rodriguez entered the school compound at 9:30 a.m. (1430 GMT) driving a gray 1993 Nissan Patrol truck, but gave no details about the explosion.
He said the truck underwent an inspection in July in the Arauco department on the border with Venezuela -- a traditional stronghold of ELN Marxist guerrillas.
The defense ministry said an investigation had been opened "to find those responsible for this terrorist act."
Ecuador's President Lenin Moreno said one of the dead was an Ecuadoran cadet, while a second suffered light injuries.
"The brutal act of terrorism in Bogota took the life of a compatriot," Moreno said on Twitter.
"My sincerest thoughts go to the family, friends and companions of Erika Chico."
Fanny Contreras, the Colombian armed forces' health inspector, told local radio that the truck "entered (the school compound) suddenly, almost hitting the police and then there was the explosion."
Carol Oviedo said her brother Jonathan, a cadet, told her on the phone he had been injured, before the connection was cut.
"In two years since he joined the police, he's never had to face a situation like this," she said.
Like other families, she was lingering in the vicinity of the academy hoping to hear some news.
For decades, residents of Bogota lived in fear of being caught in a bombing by leftist rebels or Pablo Escobar's Medellin drug cartel. But as Colombia's conflict has wound down, security has improved and attacks have become less frequent.
Rosalba Jimenez, 62, was opening her confectionary store near the school when the bomb went off.
"When we turned to look at the school the sky was grey with smoke. People were running, sirens... horrible, horrible, it seemed like the end of the world," Jimenez told AFP.
Authorities sealed off the area to the press and increased security service patrols in the south of the city, AFP reporters noted.
While authorities had yet to suggest who was behind the attack, attention was focused on leftist rebels from the National Liberation Army, which has been stepping up attacks on police targets in Colombia amid a standoff with the conservative Duque over how to re-start stalled peace talks.
The group known as the ELN was long considered a lesser military threat than the Revolutionary Armed Forces of Colombia, whose 7,000 guerrilla fighters disarmed as part of a 2016 peace accord. But in the wake of the peace deal the Cuban-inspired insurgency has been gaining strength, especially along the eastern border with Venezuela, where it has carried out a number of kidnappings and bombings of oil pipelines that have hardened Duque's resolve in refusing to resume peace talks stalled since he took office last August.
Right-wing Duque has peddled a tough line against Marxist rebels and drug traffickers in the largest cocaine producer in the world. Duque has made several demands, including the release of all hostages, as prerequisites to kick-starting the peace process, but the ELN has dismissed those as unacceptable.
After the 2016 peace accord signed by Santos and FARC guerrillas, turning the former rebels into a political party, the ELN is considered the last active rebel group in a country that has suffered more than half a century of conflict.
That cycle of violence has also involved paramilitaries, drug traffickers and other Marxist rebels, including FARC dissidents.
A year ago, six police died and 40 were injured in an attack on a police station in the Caribbean city of Barranquilla that was claimed by the ELN.
In February 2017, the ELN claimed responsibility for an attack on a police patrol in the Macarena neighborhood of Bogota that left one officer dead and several seriously wounded.
In June, three people -- including a Frenchwoman -- were killed and nine others wounded in an attack on a Bogota shopping mall that authorities blamed on a fringe left-wing group called the Revolutionary People's Movement (MRP).