The body of Gov. Luis Francisco Cuellar of Caqueta state was found not far from Florencia, the state capital where the 69-year-old was kidnapped late Monday, security official Edilberto Ramon Endo told The Associated Press.
A grim President Alvaro Uribe later went on national television to say Cuellar's throat had been cut, speaking in a sober monotone, in contrast to his anger early in the day.
"In the midst of pain we reiterate today all our determination to defeat these terrorists," he said.
Uribe said it wasn't clear when the governor was slain as 2,000 soldiers and police spread into the jungle highlands outside Florencia looking for the kidnappers.
He said senior military officials told him that "because security forces were in pursuit, the terrorists, in order to avoid gunfire, proceeded to cut the throat of the governor."
Cuellar was abducted by eight to 10 men in military uniforms who arrived at his home late Monday in a pickup, killed a police guard and blasted open the door with explosives, Gen. Orlando Paez, operations chief for the national police, told the AP. Two other police guards suffered shrapnel wounds that were not life-threatening.
The governor was driven into the mountains that border Florencia, where the pickup was abandoned and found in flames, Paez said.
The body, still in pajamas, was discovered lying at the top of a steep hill on Florencia's outskirts, said a police official who spoke on condition of anonymity because he wasn't authorizied to make public statements.
A furious Uribe, whose rancher father was killed by leftist rebels in a botched 1983 kidnapping, had ordered soldiers and police to rescue Cuellar, who was also a cattle rancher.
"We cannot continue to submit to the whims of the terrorists, of the terrorists who bathe this country in blood," Uribe told reporters earlier in the day.
Defense Minister Gabriel Silva blamed the kidnapping on the elite "Teofilo Forero" unit of the leftist Revolutionary Armed Forces of Colombia, or FARC, and announced a $500,000 reward for information leading to Cuellar's rescue.
Cuellar already had previously been kidnapped four times since 1987 not to create a political spectacle as was clearly the intent this time, but rather for ransom. His wife, Himeldo Galindo, told the AP he had been held from two to seven months in those abductions.
Caqueta has long been a stronghold of the FARC, which finances its insurgency chiefly from the cocaine trade, and is among Colombian states with the highest military presence, including an army division headquarters in Florencia.
It was in Caqueta that the FARC abducted French-Colombian presidential candidate Ingrid Betancourt in 2002 as she raced to town where government-rebel peace talks were falling apart.
That was the last year in which the FARC registered a kidnapping of a major politician, also seizing state governors and congressmen.
After Uribe was elected that year to his first term, kidnappings that had been common in Colombia's countryside sharply diminished.
The conservative president launched a full frontal assault on the FARC that he called "Democratic Security," nearly doubling the size of Colombia's military and benefiting from $700 million in annual U.S. military aid.
The July 2008 rescue of Betancourt was a triumph of Uribe's determined campaign to decimate the rebels, which has succeeded in removing guerrilla kidnappers from the country's roads and cities.
Nevertheless, Silva, the defense minister, said in a weekend newspaper interview that Latin America's largest rebel army is "neither vanquished nor in its death throes."
Though the government says the FARC has been reduced by desertions and killings to about 8,000 fighters, half its size in 2002, it continues to engage in lethal hit-and-run raids that claim several hundred lives annually. Last month, rebels killed nine soldiers in a night raid on an army post in Cauca state.
The 45-year-old FARC did not immediately take responsibility for the kidnapping. But few doubted it was behind the bold, pre-Christmas action. The FARC has a history of staging publicity-grabbing attacks at this time of year.
On Dec. 21, 1997, the rebel group killed at least a dozen soldiers and captured several others in an attack on the remote highlands outpost of Patascoy. One of those captured, Cpl. Pablo Emilio Moncayo, is now among the longest-held of 24 soldiers and police the rebels hold as bargaining chips.
Through intermediaries, the FARC has in recent weeks been negotiating Moncayo's release.
The soldier's father, Gustavo Moncayo, said Tuesday he feared Uribe was putting his son's life in jeopardy by ordering the military search for Cuellar.
"This man doesn't care about the lives of the abducted. He doesn't care a thing about the lives of the soldiers and the police," Moncayo said in a telephone interview.
The rebels, who finance their insurgency chiefly from the cocaine trade, released the last of the high-profile politicians they held in February.