LIMA (Reuters) – Six farmers were shot dead in the Peruvian Amazon by a group of masked men in an apparent dispute over land rights, a police officer and a tribal leader said on Thursday.

Five bodies, one with hands and feet bound, had been thrown in a river and a sixth was found by the side of an unpaved road in the jungle region Ucayali, police officer Raul Huari said.

The victims were part of a community of peasant farmers that had refused to leave the lands they work on when pressured by oil palm growers, said Robert Guimaraes, the head of an indigenous federation in Ucayali.

Witnesses testified that a group of between 30 and 40 men carrying shotguns tried to kill some 20 farmers altogether, Huari said.

“They said they showed up, surrounded them and just started shooting. Fortunately, some managed to escape,” he said. “In my 24 years of working I’ve never seen anything like this.”

Huari said the murders appeared to be linked to a land dispute, and a special police unit and prosecutors were carrying out investigations.

The murders occurred on Sept. 1, marking the three-year anniversary of the killing of four indigenous activists who had faced threats from loggers in a different part of Ucayali.

After the 2014 murders, which came as Peru was hosting a global environmental summit, the government promised to do more to protect remote Amazonian villages that often lack land titles from violent clashes with squatters.

Guimaraes said native communities continue to face violent threats in Ucayali and that no one has been convicted for the 2014 murders.

Peru’s culture and interior ministries did not respond to requests for comment.

The native Shipibo community of Santa Clara de Uchunya, which lives near where the farmers killed, has tried to repel oil palm growers from their lands for years, said Guimaraes.