Rescuers in hard hats and masks descended Wednesday on Mexico City in search of survivors after a deadly earthquake struck the region.

"The priority now is continue rescuing those who are still trapped and provide medical attention to the injured," President Enrique Peña Nieto said, calling Tuesday's quake -- the second to shake Mexico in 12 days -- "a new national emergency."

The magnitude-7.1 quake turned dozens of buildings in central Mexico into dust and debris, killing at least 225 people. It occurred at a depth of 51 kilometers (32 miles), which experts consider to be shallow. Shallow quakes tend to be more destructive.

"Unfortunately, many people have lost their lives, including girls and boys in schools, buildings and houses. I want to express my condolences to those who lost a family member or a loved one. Mexico shares your grief," Peña Nieto said.

According to Luis Felipe Puente, national coordinator of civil protection for the Interior Ministry, 94 of the deaths came in Mexico City, which, with an urban area of more than 21 million people, is one of the most populous cities in the Western Hemisphere.

There were also 71 deaths in Morelos state, 43 in Puebla state, 12 in the state of Mexico, four in Guerrero state and one in Oaxaca state, according to Puente's tally. To provide some scope of the affected area, Oaxaca de Juarez, the capital of Oaxaca state, is almost 480 kilometers (300 miles) from Mexico City.

The country has declared three days of mourning for those killed in the quake, according to Mexico's secretary of public function.

Twenty-five bodies, 21 of them children, were found in the debris of the Enrique Rebsamen school in Mexico City's Coapa district, Puente said in a tweet.

At least two children and one adult were still missing Wednesday, Secretary of Public Education Aurelio Nuño said on Twitter. Eleven people have been rescued from the school, he said.

The nation watched as rescuers tried to reach a girl trapped in the rubble of the collapsed school.

At the scene, the mood was somber. Those gathered around were silent, and several priests dressed in white stood off to the side.

Photos of missing students circulated on social media, while dozens of parents waited outside the school hoping to find them. The family of 7-year-old Jose Eduardo Huerta Rodriguez looked for hours through handwritten lists with the names of those who had been rescued. They also visited the city's hospitals.

Late Tuesday night, a family member who had stayed outside the school called Jose's mother.

"He was still inside the school, and he was dead when they rescued him," his aunt Paola Rodriguez told CNN.

More than 2,000 public schools suffered damage in the quake, the education secretary said. Sixteen of the 212 affected schools in Mexico City had serious damage, he said.

Four people were killed and 40 more injured at the Monterrey Institute of Technology campus in Mexico City, the school said.

An unaccounted number of people are staying at shelters around the city after losing their homes. Schools have closed indefinitely, and millions remain without power.