SANIAG - Passing motorists who had nothing to do with a political dispute were likely among 57 people killed when gunmen opened fire on a convoy of cars in the southern Philippines, police said Wednesday.

The gunmen, allegedly hired by a local politician who wanted to eliminate a challenge from a rival, stopped a convoy of six vehicles on Monday and shot the passengers from close range, according to police.

The passengers were aides and relatives of the rival politician, plus a group of journalists. Five of the convoy's blood-spattered and bullet-riddled vehicles were found on Monday alongside an unpaved farm road near many of the bodies, said Chief Superintendent Felicisimo Khu, the top police forensics official on the scene.

The final car was also found close by on Wednesday in a hastily dug grave alongside 11 freshly discovered bodies, Khu said.

But police also unearthed a sedan and a second van, neither of which was part of the convoy and which were owned by residents of the area who had no affiliation to the politicians or journalists, according to Khu. "The car and the second van were not part of the convoy," Khu said.

"They just happened to drive past."

He said relatives of the two vehicles' owners were waiting outside police lines to claim their kin, who were presumed to have been killed. But Khu could not say exactly how many people who had been in those cars had been killed.

The Philippines has become the most dangerous place in the world for journalists, according to an international press watchdog, after an election-linked massacre of at least 57 people this week.

The Belgium-based International Federation of Journalists said that by its count at least 12 journalists and "around eight media staff" were among the victims of the mass killing, blamed on a local politician and his bodyguards.

"Under the current government the Philippines has become the most dangerous place in the world for media workers," the group said in a statement, urging President Gloria Arroyo's government to give the press more protection.

"At least 74 journalists have been killed during its eight-year tenure, yet the government has not acted to end the culture of impunity. At last count, only four convictions had been secured," it said.

The group said it would send an investigative mission to the southern Philippines region of Mindanao, where the killings happened on Monday. "The Arroyo administration must make a clear and unequivocal commitment to an immediate, independent and effective inquiry into this atrocity," said the group's general secretary Aidan White.

"With elections due in six months' time the authorities must act now to guarantee the safety of journalists throughout the country." Arroyo on Wednesday vowed to hunt down the perpetrators of the massacre.