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Burned cars are pictured in a highway in Ecatepec near Mexico city, on May 7, 2013. A gas tanker exploded in a Mexico City suburb on Tuesday, killing at least 18 people and damaging several homes and cars, an official said. VICTOR ROJAS/AFP/Getty Images ( 519530455 )

Firefighters work as a house burns after a gas tanker truck exploded on the highway in front of the house in the Mexico City suburb of Ecatepec, early Tuesday, May 7, 2013. The blast killed and injured dozens, according to the Citizen Safety Department of Mexico State. Officials did not rule out the possibility the death toll could rise as emergency workers continued sifting through the charred remains of vehicles and homes built near the highway on the northern edge of the metropolis. (AP Photo/Gabriela Sanchez) ( Mexico Gas Explosion )

Aerial view shows first responders working next to destroyed homes and vehicles after a gas tanker truck exploded on a highway in the Mexico City suburb of Ecatepec early Tuesday, May 7, 2013. The blast killed and injured dozens, according to the Citizen Safety Department of Mexico State. Officials did not rule out the possibility the death toll could rise as emergency workers continued sifting through the charred remains of vehicles and homes built near the highway on the northern edge of the metropolis. (AP Photo) ( Mexico Gas Explosion )

Firefighters search for victims at the place where a gas tanker exploded in Ecatepec, Mexico state on May 7, 3013. A gas tanker exploded in a Mexico City suburb on Tuesday, killing at least 18 people, wounding 36 others and damaging several homes and cars, an official said. Ronaldo Schemidt/AFP/Getty Images ( 519534448 )

Police officers and rescue workers stand at an area damaged by the explosion of a gas tanker truck in San Pedro Xalostoc, on the outskirts of Mexico City May 7, 2013. A gas tanker truck exploded on a highway north of Mexico City early Tuesday, killing at least 18 people and injuring 36 others, the minister of public security in the State of Mexico said. REUTERS/Tomas Bravo ( MEXICO-EXPLOSION/ )

Residents look out the window of a home that was damaged when a gas tanker truck exploded on a nearby highway in the Mexico City suburb of Ecatepec, early Tuesday, May 7, 2013. The blast killed and injured dozens, according to the Citizen Safety Department of Mexico State. Officials did not rule out the possibility the death toll could rise as emergency workers continued sifting through the charred remains of vehicles and homes built near the highway on the northern edge of the metropolis. (AP Photo/Gabriela Sanchez) ( Mexico Gas Explosion )

A paramedic treats two injured dogs after a gas tanker truck exploded on a nearby highway in the Mexico City suburb of Ecatepec, early Tuesday, May 7, 2013. The blast killed and injured dozens, according to the Citizen Safety Department of Mexico State. Officials did not rule out the possibility the death toll could rise as emergency workers continued sifting through the charred remains of vehicles and homes built near the highway on the northern edge of the metropolis. (AP Photo/Gabriela Sanchez) ( Mexico Gas Explosion )

Firefighters remove a body from a destroyed home after a gas tanker truck exploded on a nearby highway in the Mexico City suburb of Ecatepec early Tuesday, May 7, 2013. The blast killed and injured dozens, according to the Citizen Safety Department of Mexico State. Officials did not rule out the possibility the death toll could rise as emergency workers continued sifting through the charred remains of vehicles and homes built near the highway on the northern edge of the metropolis. (AP Photo/Gabriela Sanchez) ( Mexico Gas Explosion )

Firefighters work next to destroyed houses and vehicles after a gas tanker truck exploded on a nearby highway in the Mexico City suburb of Ecatepec early Tuesday, May 7, 2013. The blast killed and injured dozens, according to the Citizen Safety Department of Mexico State. Officials did not rule out the possibility the death toll could rise as emergency workers continued sifting through the charred remains of vehicles and homes built near the highway on the northern edge of the metropolis. (AP Photo/Marco Ugarte) ( Mexico Gas Explosion )

Aerial view shows emergency responders standing next to a part of a tanker truck after it exploded and landed in a residential area near a highway in the Mexico City suburb of Ecatepec early Tuesday, May 7, 2013. The blast killed and injured dozens, according to the Citizen Safety Department of Mexico State. Officials did not rule out the possibility the death toll could rise as emergency workers continued sifting through the charred remains of vehicles and homes built near the highway on the northern edge of the metropolis. (AP Photo) ( Mexico Gas Explosion )

Miryan Lopez Zedillo (C), a relative of victims of an explosion, cries beside other residents near at the site of the explosion of a gas tanker truck in San Pedro Xalostoc, on the outskirts of Mexico City May 7, 2013. A gas tanker truck exploded on a highway north of Mexico City early on Tuesday, killing at least 20 people and injuring 36 others as a fireball tore through cars and homes. REUTERS/Henry Romero ( MEXICO-EXPLOSION/ )

Residents stand on a charred rooftop in the Mexico City suburb of Ecatepec, where a gas tanker truck exploded on the highway, early Tuesday, May 7, 2013. The blast killed and injured dozens, according to the Citizen Safety Department of Mexico State. Officials did not rule out the possibility the death toll could rise as emergency workers continued sifting through the charred remains of vehicles and homes built near the highway on the northern edge of the metropolis. (AP Photo/Gabriela Sanchez) ( Mexico Gas Explosion )

Two bodies covered in sheets lie in a room of a destroyed home after a gas tanker truck exploded on a nearby highway in the Mexico City suburb of Ecatepec early Tuesday, May 7, 2013. The blast killed and injured dozens, according to the Citizen Safety Department of Mexico State. Officials did not rule out the possibility the death toll could rise as emergency workers continued sifting through the charred remains of vehicles and homes built near the highway on the northern edge of the metropolis. (AP Photo/Gabriela Sanchez) ( Mexico Gas Explosion )

A charred bird cage hangs from the wall of a home destroyed when a gas tanker truck exploded on a nearby highway in the Mexico City suburb of Ecatepec, early Tuesday, May 7, 2013. The blast killed and injured dozens, according to the Citizen Safety Department of Mexico State. Officials did not rule out the possibility the death toll could rise as emergency workers continued sifting through the charred remains of vehicles and homes built near the highway on the northern edge of the metropolis. (AP Photo/Gabriela Sanchez) ( Mexico Gas Explosion )

Residents search amid the wreckage and ruins left by the blast of a gas tanker that exploded in Ecatepec, Mexico state on May 7, 3013. A gas tanker exploded in a Mexico City suburb on Tuesday, killing at least 18 people, wounding 36 others and damaging several homes and cars, an official said. YURI CORTEZ/AFP/Getty Images ( 519537962 )

Broken windows outside a classroom are seen near the site of the explosion of a gas tanker truck in San Pedro Xalostoc, on the outskirts of Mexico City May 7, 2013. A gas tanker truck exploded on a highway north of Mexico City early on Tuesday, killing at least 20 people and injuring 36 others as a fireball tore through cars and homes. REUTERS/Henry Romero ( MEXICO-EXPLOSION )