Colombia’s police force has suspended four officers who fired into a crowd of protesters in a confused incident that left at least six farmers dead and 20 others wounded in the south-western region of Tumaco.

The move comes a day after police in the area fired into the air and launched stun grenades against a humanitarian commission made up of human rights activists and local journalists as they attempted to reach the remote site of the killings.

President Juan Manuel Santos on Monday called the episode “regrettable” and promised a thorough investigation of both incidents, which have highlighted the continued threat of violence in the country even after a peace deal with Farc rebels.

The six farmers were killed on 5 October during a protest by growers of coca leaf, used in making cocaine, against forced eradication efforts. Security forces first blamed a dissident faction of the Farc for the deaths but local activists said the police had opened fire on the protesters. Preliminary forensic testing showed the six farmers were killed by high-velocity long-range bullets.

Facebook Twitter Pinterest President Juan Manuel Santos, left, and Vice-President Oscar Naranjo. ‘The immense majority of the testimony signals the police as responsible,’ Naranjo said. Photograph: John Vizcaino/AFP/Getty Images

By Monday the government acknowledged that witnesses overwhelmingly blame police. “The immense majority of the testimony signals the police as responsible,” said Vice-President Óscar Naranjo, a former chief of national police who traveled to Tumaco after Thursday’s killings.

Although government officials have promise a full investigation, Naranjo did not rule out the possibility that the scene of the killings might have been tampered with. “It was unprotected for 12 hours,” he said. “We have to be realistic.”

Speaking to RCN Radio, Naranjo repeated the claim that the protesters had been ordered by the dissident guerrillas to confront security forces escorting a group of civilians carrying out coca eradication efforts.

Tumaco, a sprawling municipality on Colombia’s Pacific coast, has the highest concentration of coca crops in the country, highly productive drug labs and well-traveled routes to export cocaine, attracting organized crime groups and dissident rebels from the former Daniel Aldana front of the Farc.

The Farc, which signed a peace deal last year to end its more than 50-year war against the state, long promoted the growing of coca in the region and used the drug trade as a means to finance its fight. In Tumaco the group attracted farmers from other parts of the country to occupy land held communally by Afro-Colombian communities. The presence of the “colonists” sparked bitter feuds between the local and migrant communities.

Speaking by telephone from Tumaco, a member of the Afro-Colombian Community Council of Alto Mira and Frontera – who asked to remain anonymous for security reasons – said council leaders were under “death threat from drug traffickers for participating in crop substitution” and for resisting pressure to participate in the anti-eradication protests.

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But even those who have signed up to voluntarily substitute coca for legal crops, as part of the peace deal with the Farc, complain that they have seen their coca fields destroyed without any accompanying government compensation or support. “They don’t have the money to support us and the pressure to continue is fierce,” the community leader said.

Colombia is producing more cocaine than ever before, according to United Nations data. A government program to manually eradicate the 188,000 hectares of coca crops calls for voluntary eradication through pacts with communities and, where that fails, forcibly pulling up the plants.

Anti-narcotics police were preparing to forcibly eradicate crops in Tumaco when the clashes began.

The UN mission in Colombia, which is overseeing the implementation of the peace deal with the Farc, said the incident underscored the need for swift action. “The events reinforce our conviction at the United Nations of the necessity of giving coca farmers in affected regions all the means to escape the terrible choice between extreme poverty and illegality,” Jean Arnault, the UN mission chief, said.

Amnesty International said the events in Tumaco show the need for full implementation of the peace accord with the Farc. “Nothing will change in Colombia until everyone in the country can protest peacefully without fearing for their lives,” said Erika Guevara-Rosas, Amnesty’s director for the Americas.