President Alvaro Uribe defied alleged rebel threats on his life Saturday to visit the southern city where a powerful bomb killed 18 people the day before by ripping through a house as police searched for weapons.

A somber Uribe offered condolences to relatives of those killed in the blast, which authorities said may have been caused by explosives intended for a plot to kill the president during his weekend visit to the city.

The rebels either were planning to fire mortars from a house in the flight path to the airport in Neiva when Uribe's plane passed overhead, or detonate the bomb in the hope that the explosion would be strong enough to down the plane, authorities said.

Instead, police were tipped off to the threat and raided the house. It was not immediately clear what triggered the bomb, which also injured dozens of people and destroyed four houses.

On Saturday, cleanup crews swept glass, concrete, burned clothing and shoes from the scene, where the bomb gouged a 15-foot hole in the ground. One resident said he feared Uribe's presence in Neiva would only make things worse.

"It's a provocation for the guerrillas, and we're the ones who will pay," said Alfredo Vargas, a teacher, standing near a pile of debris and two concrete walls that once were part of his home.

The dead included three children, and nine police officers and a government investigator who were searching the house when the bomb exploded.

The government blamed the explosion on the leftist Revolutionary Armed Forces of Colombia, or FARC, the country's largest rebel army. The FARC has been fighting a series of elected governments for 38 years.

Uribe visited victims' relatives at a funeral home and hospital and called the rebels "cowards." He was expected to end Saturday's meeting early to attend a funeral for victims.

Elsewhere in southern Colombia, hundreds of soldiers, aided by U.S.-donated Black Hawk helicopters, scoured rebel territory Saturday for three Americans allegedly kidnapped by the FARC after their plane crashed while on an intelligence mission.

Rebels shot and killed a fourth American and a Colombian army sergeant who also were on the plane, said Gen. Jorge Mora, the country's top military commander.

Uribe, a hard-liner who has vowed to subdue the rebels, has survived more than a half-dozen assassination attempts during his decades-long political career. During last year's presidential campaign, several bystanders were killed when a bomb exploded as Uribe's motorcade passed by in the northern city of Barranquilla.

Suspected rebels also fired mortars in the direction of the national congress while Uribe took the oath of office in August. The mortars hit a nearby slum and working-class neighborhood, killing 21 people.

Since taking office, Uribe has raised taxes and lobbied Washington to help fight the rebels. He also has recruited peasant soldiers and civilians to assist Colombia's military in its fight against the insurgents.

Colombia in recent days has been hit with some of its worst violence in recent memory. On Feb. 7, a car bomb allegedly planted by the FARC blew up in an upscale social club in Bogota, killing 35 people and injuring more than 100 others.