QUITO, Ecuador (AP) — Ecuador’s transformation into a major drug trafficking hub and the ensuing three-year surge of violence is weighing on the nation following the killing of a presidential candidate whose life’s work was to fight crime and corruption.

Six Colombian men were arrested Thursday in connection with the fatal shooting of Fernando Villavicencio a day earlier in the capital, Quito. He was not a front-runner in the race, but his assassination in broad daylight less than two weeks before the special presidential election underscored the challenge Ecuador’s next leader will face in any attempt to curb gangs and cartels whose activities have claimed thousands of lives.

A report of the men’s arrest reviewed by The Associated Press showed the men were captured hiding in a house in Quito. Law enforcement officers, according to the report, seized four shotguns, a 5.56-mm rifle, ammunition and three grenades as well as a vehicle and one motorcycle.

Ecuador’s Interior Minister Juan Zapata described the killing as a “political crime of a terrorist nature” aimed at sabotaging the Aug. 20 election. The police report doesn’t say whether the Colombians are members of a criminal group. But Zapata, who confirmed the arrests of some foreigners without giving their nationalities, said the suspects were linked to organized crime.

Villavicencio, 59, had said he was threatened by affiliates of Mexico’s Sinaloa cartel, one of a slew of international organized crime groups that now operate in Ecuador. He said his campaign represented a threat to such groups.

With almost 400 miles of Pacific coast, shipping ports and some key exports, Ecuador has been turned by international traffickers from a minor player in the drug business into a big regional hub for the smuggling of cocaine.

“Ecuador has the geographical misfortune of being sandwiched between Colombia and Peru, the two largest cocaine producers in the world, and underlying it all is a certain degree of institutional weakness in the judiciary, police and military,” said Cynthia Arnson, a distinguished fellow at the Washington-based Wilson Center and an expert in Latin America.

She added the killing shows “criminal actors most likely connected to organized crime in Ecuador feel that they can act with impunity, going as far as to assassinate an anti-corruption political candidate.”