Quito, Ecuador - Ecuadorian President Guillermo Lasso on Thursday said he has asked the FBI is to assist with the investigation into the killing of a candidate in the upcoming presidential elections.

"I have asked the FBI for assistance in the investigation into the assassination of presidential candidate Fernando Villavicencio Valencia," Lasso said on social media. "The FBI has granted our request and a delegation will arrive in the country in the next few hours."

Opposition candidate Villavicencio was shot dead after an election rally in the capital Quito on Wednesday. At least nine other people were injured. One of the suspected attackers was wounded in a shoot-out with police and later died on the way to the hospital.

The six people arrested in connection to the shooting are reportedly Colombian nationals.

Lasso, who is not running for president himself, declared a 60-day state of emergency, a three-day nationwide state of mourning and mobilized troops in response to the mafia-style killing.

Early presidential and parliamentary elections will be held in Ecuador on 20 August. These had become necessary after Lasso dissolved the National Assembly amid impeachment proceedings for alleged embezzlement.

The country is suffering from an unprecedented wave of violence. The murder rate of 25 homicides per 100,000 inhabitants last year was the highest in its history.

The government holds drug traffickers primarily responsible.