A blast which killed four people during carnival celebrations in the festive Bolivian city of Oruro was caused by a bomb, police said Wednesday.
Police commander Faustino Mendoza said a device packed with three kilos of dynamite exploded late Tuesday, killing bystanders.
"We have four people killed and nine injured," said Mendoza, adding that the blast occurred in a street near the scene of an even deadlier explosion on Saturday, which killed eight people.
Initially, a government minister said that explosion was likely caused by a street vendor's gas cylinder.
Following the second blast, regional governor Victor Hugo Vasquez said: "We are sure that this is a criminal attack."
Defense Minister Javier Zabaleta said authorities were now considering "the possibility that the two explosions are coordinated events."
President Evo Morales, in a statement on Twitter, said the South American country had been plunged into mourning.
"Very saddened and concerned because we have a tragedy again, and the death of innocent bystanders in Oruro has our country in mourning after another explosion," he wrote.
The northwestern Bolivian city is known for its cobblestone streets and its colorful carnival celebrations that attract as many as half a million visitors to the city.