MEXICO CITY (AP) — A gas tanker truck exploded on a highway in the Mexico City suburb of Ecatepec early Tuesday, killing at least 18 people and injuring three dozen, according to the Citizen Safety Department of Mexico State, which surrounds Mexico City.

Officials did not rule out the possibility the death toll could rise more as emergency workers continued sifting through the charred remains of vehicles and homes built near the highway on the northern edge of the metropolis.

A huge piece of the truck’s gas tank was blown a hundred yards by the force of the blast, landing atop the wall of a house and cars parked outside. Charred wreckage of cars littered the blast site.

Hundreds of police, ambulances drivers and paramedics, soldiers and firefighters gathered at the scene, where a giant plume of smoke rose over the area after the explosion around 5 a.m. Tuesday local time.

The explosion closed the highway between Mexico City and Pachuca.

The pre-dawn accident exposed two recurrent public safety issues in Mexico: extremely heavy trucks that are frequently involved in serious accidents, and the construction of improvised homes just feet away from major highways.

Some of the cinderblock homes hit by the massive explosion were just steps away from the busy, six-lane highway.

Mexican trucks, often overloaded or unsafely operated, have been involved in a number of spectacular, deadly accidents in recent years.

One year ago, the Mexican government announced measures to tighten inspections and lower maximum allowed weights for freight trucks after protests over a string of deadly accidents involving double-trailer trucks.

Mexico had allowed trucks to travel two-lane roads with loads of up to 80 metric tons and lengths exceeding 100 feet, compared to a U.S. limit of is 80,000 pounds (40 tons) on interstate highways. It subsequently reduced that limit by about 4.5 tons.

In April 2012, a double-trailer truck on a two-lane road in the Gulf coast state of Veracruz lost its rear trailer, which slammed into a bus carrying farm workers, killing 43 people.