The six people who have been arrested in connection with the killing of Ecuadorian presidential candidate Fernando Villavicencio are Colombian, police say.
A seventh suspect, who died from wounds in a shootout with police on Thursday, was also Colombian.
Mr Villavicencio was killed leaving a campaign event in the capital Quito.
The interior minister said a police investigation into the "abominable event" was under way.
Juan Zapata said officers would work to "discover the motive of this crime and its intellectual authors".
A vocal critic of organized crime, Mr Villavicencio was one of the few presidential candidates to allege links between corruption and government officials.
President Guillermo Lasso said the assassination was an attempt to sabotage the election.
He added that voting would go ahead as planned on 20 August, despite a national state of emergency.
He said organised crime was behind the killing and has asked US federal agents to help investigate, with FBI agents due to arrive shortly.
Mr Villavicencio, a member of the country's national assembly, had received threats from a gang calling itself Los Choneros last month and had been given a security detail.
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Following his murder, a video appeared on social media in which heavily armed men wearing balaclavas claimed responsibility for the murder. The men claimed to belong to Los Lobos (The Wolves), who are rivals of Los Choneros.
However another video appeared online just hours later, in which another group of men – this time not wearing masks – claimed they were Los Lobos members and denied any role in the assassination, claiming the other video was an attempt by their rivals to set them up for the murder.
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Media caption,
Fernando Villavicencio was shot as he left a rally in Quito
Ecuador: The basics
Where is Ecuador?
It's the smallest of the Andean nations in South America, sitting on the equator (hence the name) between Colombia and Peru.
Why was Mr Villavicencio shot?
He was one of eight candidates in the running for the first round of the election with a focus on fighting corruption – and he and his team had been threatened by the leader of a gang linked to drug-trafficking.
What next?
Once a relatively peaceful nation, Ecuador has been ravaged by the arrival of international drug cartels profiting from a boom in cocaine trafficking – and the issue can only grow in importance in the presidential election campaign.