Agents from the US Federal Bureau of Investigation (FBI) were meeting Ecuadorean police and prosecutors on Sunday as part of a joint effort to uncover who was behind last week's assassination of presidential candidate Fernando Villavicencio.

The killing of the 59-year-old in the closing days of the campaign has sent shockwaves through the South American country of 18 million, where violent crime stoked by transnational criminal gangs has risen sharply in recent years.

Villavicencio, an ex-lawmaker and investigative journalist with a track record of exposing corruption, repeatedly said he was not afraid of the gangs despite receiving threats.

On Sunday, Interior Minister Juan Zapata told reporters the FBI team had already met police leaders and would in the "next few hours" meet prosecutors from the attorney general's office who are leading the investigation into Villavicencio's slaying.

Six Colombian nationals have been charged with the murder and remain in custody while one other suspect died after an exchange of gunfire shortly after the murder. Police accuse the Colombians of ties to criminal groups.

Villavicencio was one of eight candidates crisscrossing the Andean country for votes ahead of the August 20 election.

Outgoing President Guillermo Lasso asked for FBI help in the case on Thursday, the day after Villavicencio was shot multiple times as he stepped into a car surrounded by a small group of people that included his own government-provided bodyguards.

Earlier on Sunday, leaders of the Build party, or Construye in Spanish, announced they would now opt for Christian Zurita to replace Villavicencio as the party's top candidate, reversing their decision from Saturday to elevate the party's vice presidential nominee.

Zurita is also a journalist who in the past collaborated with Villavicencio. His candidacy must still be approved by the national electoral council. (Reuters)