Nine people were killed Thursday and more than 50 wounded in a car bomb attack on a police cadet training academy in the Colombian capital Bogota.
The defence ministry said the "terrorist act" -- using a vehicle packed with around 175 pounds (80 kilograms) of explosives -- was the country's worst since a peace accord was signed with FARC rebels more than a year ago.
"All Colombians reject terrorism and we're united in fighting it," President Ivan Duque tweeted.
Vowing to bring to justice those responsible, Duque added: "COLOMBIA is sad but will not bow to violence."
The bomber -- who was 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."
A security council meeting that Duque was due to attend outside the capital was cancelled, with the president heading immediately back to Bogota.
The defence ministry said an investigation had been opened "to find those responsible for this terrorist act," which left 54 people in treatment at four hospitals, according to the health secretariat.
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 to the families, friends and companions of Erika Chico."
Fanny Contreras, the armed forces' health inspector, told local radio the car "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.
United States assistant secretary of state in charge of Latin America, Kimberly Breier condemned the attack and said: "Our condolences and sympathies go to the victims and family members of those killed."
Rosalba Jimenez, 62, was opening her confectionary store near to the school when the bomb went off.
She told AFP she thought the explosion came from a petrol station nearby but "when we turned to look at the school the sky was gray with smoke. People were running, sirens... horrible, horrible, it seemed like the end of the world."
Authorities sealed off the area to the press and increased security service patrols in the south of the city, AFP reporters noted.
Right-wing Duque, who assumed power in August, has peddled a tough line against Marxist rebels and drug traffickers in the largest cocaine producer in the world.
Peace talks with ELN guerrillas -- who in the past have claimed responsibility for bomb attacks on police -- stalled before Duque replaced Santos as president and have not been restarted.
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.
Since the 2016 peace accord between Santos and FARC guerrillas, turning the former rebels into a political party, the ELN remains the last recognized armed 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.
Bogota suffered a pair of major attacks in 2017.
In February of that year, the ELN claimed responsibility for an attack on a police patrol in the Macarena neighborhood 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 shopping mall that authorities blamed on a fringe left-wing group called the Revolutionary People's Movement (MRP), which had previously been accused of carrying out low-impact attacks in the capital.