Updated

Children returned to school Thursday and nervous adults tried to get back to normal after being stunned by two deadly explosions during this mining city's exuberant Carnival celebrations.

People remained on edge as authorities sought explanations for the two blasts in normally peaceful Bolivia. A crater gouged into the ground at one site and splintered chairs scattered on a nearby street provide grim reminders of the 12 people killed and 50 injured.

Police have blamed a bomb made of dynamite, ammonium nitrate and fuel oil for a Tuesday night blast that killed four.

After the blast, officials said they were re-examining an explosion two blocks away that left eight dead late Saturday and was initially blamed on a food vendor's leaking gas canister. Local prosecutor Mario Rocha said Thursday that investigators had not found any pieces of a gas canister.

As residents called for answers, officials sought to determine if the blasts were connected and who might have been responsible for the explosions in Oruro, which is the capital of President Evo Morales' home province. Three people detained for questioning after Tuesday night's explosion were released Wednesday.

"Nothing has been ruled out, not even a political motive," said Rocha. "We're still carrying out the investigations and there is no deadline. There are no arrests."

A political attack would be a startling development for modern Bolivia. No guerrilla or major violent groups operate in the Andean country of 11 million people, where violence is mostly confined to drug dealer turf wars. The last bombings occurred more than 35 years ago during a 1980-1981 dictatorship.

"It's strange and atypical ... in a country that is not used to bloody acts," said Franklin Pareja, a professor of political science at Public University of La Paz.

People in a poor neighborhood held a wake Thursday for 3-year-old Alex Quekana, who was killed by Tuesday's explosion. His father, mother and a brother are hospitalized for serious injuries.

"We're poor. We're innocent. Our whole family has been destroyed," said a grandmother, Maxima Solares. "If there's no justice, I will demand it from Evo Morales."

The boy's mother, Demetria Massi, had not been told her toddler son died.

Oruro is about 135 miles (220 kilometers) southeast of La Paz, and its Carnival draws thousands of people who dance through the streets in elaborate, colorful costumes while brass bands blare.

Street vendors were offended when authorities blamed the first explosion on a food vendor's gas canister.

"That's not true. We think it was an attack," said Elizabeth Herrera, a street vendor whose daughter and two of her granddaughters were killed by the first blast.

"We're the victims," she said.

The crater left by the second blast has been cordoned off by police, but the streets remain largely unpatrolled and residents worry about the safety of their loved ones as the city gets back into its normal routines.

"Who did this?" Rosario Roque asked near the site of Tuesday night's explosion. "I walk by the streets every day. Now, we're scared to send our children to school."