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Brugada said prosecutors were investigating, but it appeared that the truck exploded after it tipped over on the highway.

THIS IS A BREAKING NEWS UPDATE. AP’s earlier story follows below.

MEXICO CITY (AP) — A gas tanker truck exploded along a Mexico City highway Wednesday, sending flames and smoke billowing over the south of the capital and injuring at least 57 people, some with their entire bodies charred and others waiting for help in the middle of the road with burns and torn clothing.

Mayor Clara Brugada called the explosion under a highway overpass an “emergency" that burned 18 vehicles and left 19 of those injured in grave condition. She said no one had yet died from their injuries, and the wounded were taken to hospitals across the city.

Brugada said prosecutors were investigating the incident, but it appeared that the truck exploded after it tipped over on the highway.

Images circulated online by authorities showed a mass of flames coming from a truck, while other videos on social media show dozens of people screaming and running from the explosion. The videos show two men whose entire bodies appeared to have suffered burns, with tattered clothing melded onto skin, as emergency vehicles sped by.

Cesar Cravioto, the government secretary of Mexico City, said the fire was "completely under control.�

The gas tanker laying on its side had the logo of the energy business Silza on its side, but in a call with the Associated Press an official at the company who did not want to be identified company denied that it was their vehicle, saying they only operate in northern Mexico. The company did not immediately respond to an email requesting more details.

Brugada arrived to the scene of the explosion shortly after to accompany emergency teams, including firefighters and medics. Fire fighters were pouring water on the flames billowing from the gas tanker.

The explosion occurred on one of the most important roadways flowing out of the country's capital on the way to the city of Puebla. After hours of closures, the city's government reopened the roadway on Wednesday evening.