Colombia is digging up the remains of a former guerrilla leader who was assassinated 25 years ago while running for president in one of the most emblematic crimes committed during a wave of terror against leftist activists.

Carlos Pizarro was killed when an assailant opened fire with a machine gun inside a crowded airliner in midflight.

Authorities on Wednesday dug up Pizarro's remains and those of his assailant to resolve one of the crime's nagging doubts: whether bodyguards assigned by the state to protect the politician also had a hand in his murder.

"There's evidence in the case that doesn't add up," Maria Jose Pizarro, the slain politician's daughter, told The Associated Press as forensic specialists removed the marble tombstone covering the remains in Bogota's Central Cemetery. "It suggests there's a campaign to stop any progress from being made in the investigation."

The former M-19 rebel leader was the third candidate killed during the 1990 presidential race. At the time, leftists and anti-corruption crusaders were being hounded by right-wing paramilitaries aligned with powerful drug cartels.

Evidence that has surfaced in other high-profile crimes from the dark era suggest that corrupt state agents were frequently active participants in the dirty war.

"There's lots of questions about how the weapon got on board and if the only shots that were fired came from the assassin," said Sergio Reyes Blanco, a representative from Inspector General's office who is investigating the murder.

Pizarro was born into a wealthy Bogota family and, inspired by Cuba's communist revolution, joined the M-19 guerrillas. The group later demobilized as part of a peace agreement and formed a political party that nominated Pizarro as its presidential candidate. He was 38 at the time of his murder.