A massive explosion ripped through a house Friday as it was being searched by police investigating a plot to kill President Alvaro Uribe, killing 16 people and scattering debris for blocks in this southern Colombian city.

The pre-dawn explosion destroyed three other houses in the working-class neighborhood adjoining the airport. Authorities said among those killed were nine police officers, an investigator with the attorney general's office and three children.

If there were any doubts that Colombia's four-decade-old war had moved into this South American nation's cities, they were erased in the devastating blast that gouged a 15-foot-deep crater in the ground and turned a quiet neighborhood into a scene of devastation.

Security officials said the rebels of the Revolutionary Armed Forces of Colombia, known as the FARC, planned to kill the president on Saturday as he flew to Neiva, a city of some 250,000 residents about 250 miles south of Bogota.

The rebels allegedly planned to kill Uribe by detonating the bomb as his plane flew low overhead -- the house was in the flight path -- or by firing mortar rounds, authorities said.

Also Friday, Uribe said that an American and Colombian whose bodies were found in the wreckage of a U.S. government plane had been murdered. Authorities feared three other Americans on the single-engine Cessna were being held by the FARC.

amerSome 85 miles to the south, in the city of Florencia, officials said the bodies of an American and a Colombian found in the wreckage of a U.S. government plane had gunshot wounds.

The Cessna went down Thursday in rebel territory near Florencia, about 85 miles south of Neiva. It was unclear if the two men, who had been shot, died from gunfire or from the crash.

Florencia and Neiva are two of the largest cities on the border of a huge safe haven that the Colombian government granted to the FARC at the end of 1998 as a site for peace talks.

Tipped that there might be explosives and mortar tubes in four houses in Neiva, police and the investigator from the federal prosecutor's office raided the homes at 5:30 a.m.

They were told they would find material being prepared for an assassination attempt against the president, said Luis Alejandro Gomez, police commander in Huila state. In two of the houses, they found detonating devices and pieces of metal. In the third, they found nothing. Officials still aren't sure what was found in the house that exploded.

The explosion was heard throughout the city and scattered debris for 15 blocks. Dozens of houses were heavily damaged.

"My God! My God," exclaimed Irma Diaz, whose house disappeared in the blast. "Every day, something worse happens."

Scattered on the earth were the items that she and her neighbors had scrimped and saved for, now turned to debris: children's shoes, pieces of tables, carpets, pots and pans, a set of keys to doors that no longer existed.

Diaz, 50, said she rushed home from her cleaning job after hearing the explosion to check on her daughter. The girl was alive but suffered a fractured hip.

Her neighbor, Nancy Castro, was sleeping alongside her husband when the bomb blast stopped her clock at 5:25 a.m. and sent roof tiles cascading onto the couple.

"When I opened the door, there was only debris," Castro said. "The first thing I saw was a little girl in a white dress drenched with blood."

The government canceled peace talks with the FARC on Feb. 20, 2002, after the rebels hijacked an airliner and kidnapped a senator on board. Many Colombians braced for a bloodier war.

A year later, it has come. The FARC also set off a huge bomb on Feb. 7 in an exclusive club in Bogota, the capital, killing 35 people, wounding more than 100 and turning the 11-story building into a charred hulk.

The rebels insisted in a communiqu'e this week that the government grant another safe haven like one they had until the peace talks were canceled. Uribe, a hard-liner elected by a landslide last year, rejected the proposal as "terrorists' buffoonery."

Agents of the U.S. Bureau of Alcohol, Tobacco and Firearms, meanwhile, completed their investigation of the bombing of the El Nogal Club in Bogota. They determined the bomb was packed into a Renault that had been parked in an interior garage on a floor directly underneath squash courts, a restaurant and a bar, in order to cause maximum casualties.

The explosive charge weighed 330-440 pounds and had been composed of a mixture of ammonium nitrate, fuel oil and TNT. The blast was so powerful it blew some pieces of the Renault five blocks away.

Separately, counterterrorist police in the city of Medellin seized half a ton of explosives Friday along with rocket launchers and arms and arrested 12 suspected members of the National Liberation Army, or ELN, Colombia's second-largest rebel group.

"We have never seized such a quantity of explosives," said National Police Chief Teodoro Campo, in a statement released by the presidency.

The FARC and the National Liberation Army have fought the government for nearly 40 years. About 3,500 people, mostly civilians, die each year in the fighting.