TULTEPEC, Mexico (Reuters) - Forensic investigators scoured the charred remains of a fireworks market outside Mexico City on Wednesday for clues to what caused a series of massive blasts that killed at least 33 people, the third fiery accident there in 11 years.

Dozens of people were injured in Tuesday’s disaster at the San Pablito open-air market, which was crowded with shoppers just before Christmas.

A smell of burning hung over the remains of the market where investigators dressed in white protective gear, police, and medical personnel searched through twisted metal frames and the wreckage of stalls. Soldiers with dogs appeared to be looking for human remains.

Alejandro Gomez, the state attorney general, told Mexican television it was unclear what caused the explosions, adding he could not corroborate accounts pointing to a detonation at one stall that may have begun a chain reaction.

Video of the blasts showed a spectacular flurry of pyrotechnics exploding high into the sky, like rockets in a war zone, as a massive plume of charcoal-gray smoke billowed out from the site.

The tragedy could weigh on the gubernatorial election in the State of Mexico next summer, where the ruling Institutional Revolutionary Party (PRI) aims to hold on to the populous region after a slump in President Enrique Pena Nieto’s popularity.

The federal attorney general’s office opened an investigation, saying that there were six separate blasts.

Alberto Teres, owner of one of few stalls remaining, saw the flames leap from stall to stall. “The only thing you could do was run,” he said.

“It’s a catastrophe,” said Guadalupe Sanchez from nearby Cuautitlan Izcalli, as she searched on Wednesday morning for her uncle, 52, who owned a market stall, and two nephews, aged 15 and 9.

It was the third time in just over a decade that explosions have struck the popular marketplace in Tultepec, home to the country’s best-known fireworks shopping and about 20 miles (32 km) north of Mexico City in the State of Mexico.

In late 2005, explosions struck the market days before Independence Day celebrations, injuring scores of people. Another explosion gutted the area again almost a year later.

A sign reading “Tultepec, Firework Capital” stood at one of the exits to the market. It was particularly full on Tuesday as many Mexicans buy fireworks to celebrate Christmas and the New Year.

Burned out cars with the paint peeled off and windows punched out by the force of the blasts ringed the site.

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“Everything was destroyed, it was very ugly and many bodies were thrown all over the place, including a lot of children. It’s the worst thing I’ve ever seen in my life,” said housewife Angelica Avila, 24, tears running down her face.

Avila spoke outside a nearby hospital on Tuesday night as she waited for an update on the health of her brother, a fireworks salesman, who she said was burned and also suffered a heart attack.

Ten of the dead have yet to be identified, according to a state government website.

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State interior minister Jose Manzur said the vast majority of the market’s 300 stalls were completely destroyed. However, he noted that the site was inspected by safety officials just last month and that no irregularities were found.

The botched investigation into the disappearance and apparent massacre of 43 student teachers in 2014 in the violent state of Guerrero was a major embarrassment for Pena Nieto’s administration, which has also been criticized for its handling of probes into deadly accidents at state oil company Pemex.

The PRI controls the State of Mexico, but not the town of Tultepec, which is run by the leftist opposition, and State Governor Eruviel Avila accused the town’s mayor of seeking to use the tragedy for political ends.