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A gas truck exploded Thursday and leveled part of a maternity and children's hospital in Mexico City. Authorities said that three people, including two children, were killed and that at least 70 other people were injured, 17 of them seriously.

Authorities said Thursday night that nine children were among the more seriously injured victims. About 100 people were in the hospital at the time of the explosion, which authorities said was caused by a leak in a hose from the truck, which was fueling the hospital's tanks. Three gas tanker workers who tried but failed to fix the leaking hose have been detained, police told NBC News.

Pablo Armando Reyes Olivares, who owns a locksmith shop nearby, told NBC News that he helped remove eight newborns who survived.

"There were moms who had recently given birth who were asking for their newborns," he said. "We are all destroyed."

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Ioan Grillo, a correspondent for Global Post, described a scene of "complete chaos outside the hospital right now" — a crush of rescue crews, soldiers, police, ambulances and distraught relatives of mothers and children inside.

Mexico City Mayor Miguel Angel Mancera told the Televisa network that many of the injured were hit by flying glass. Television images showed much of the structure shattered, with a column of smoke rising and at least one fire burning in the rubble. A nearby hospital asked for blood donations.

"There was a super explosion, and everything caught on fire," Ismael Garcia, who lives a block away, told The Associated Press.

President Enrique Peña Nieto said on Twitter that crews were conducting search and rescue. Martinez said that some firefighters were overcome by gas fumes.

"My sadness and solidarity to the injured and to the relatives of those who lost their lives this morning," the president said.

Jose Luis Vazquez Sanchez, who lives about 50 yards from the hospital, said the explosion was strong enough to shatter his windows. He said that inside the hospital “there were cribs, newborns, merchandise,” and now it's "deshecho," or destroyed.

The Mexico City government put out a notice on Twitter that people should avoid the area. Police put barricades around the hospital.

Juanita Ceballos, Sandra Lilley and Emmanuelle Saliba of NBC News, The Associated Press and Reuters contributed to this report.