AMPATUAN, MAGUINDANAO—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 Maguindanao, 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 Supt. Felicisimo Khu, the top police forensics official on the scene.

The final car was also found close by 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 nor 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.