(CNN) Rescuers tunneled through debris for hours Wednesday, listening for the breathing and cries of survivors after a deadly earthquake struck Mexico City and the region.

At a collapsed school, where the bodies of nearly two dozen school children were found, teams made contact with a trapped girl. She was alive.

"The priority now is continue rescuing those who are still trapped and provide medical attention to the injured," President Enrique Peña Nieto said, callingTuesday'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 250 people, Mexico City Mayor Miguel Angel Mancera told CNN affiliate Foro TV.

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, 100 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 69 deaths in Morelos state, 43 in Puebla state, 13 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, according to Mexico's secretary of public function.

Schools hit hard

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Twenty-five bodies, 21 of them children , were discovered 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 late Wednesday as rescuers made contact with the girl trapped in the rubble of the collapsed school. The rescue teams gave her oxygen and water, Foro TV reported. The girl "feels" two other students were close to her, but she didn't know whether they were alive, Foro TV reported.

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, 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.

Search and rescue teams pulled one person from a collapsed building in the Lindavista neighborhood of Mexico City, Interior Secretary Miguel A. Osorio Chong tweeted.

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.

The front of the elementary school where rescues are still ongoing. It's somber and quiet. No one is chatting or talking. The police are very serious and the most telling of all, the priests who are waiting to the side. @cnn #mexicocity #earthquake A post shared by Khushbu Shah (@khushbuoshea) on Sep 20, 2017 at 11:35am PDT

'People screaming, children everywhere'

Photos: Deadly earthquake rocks central Mexico Relatives of missing people wait for news in front of a collapsed building in Mexico City on Friday, September 22. A magnitude 7.1 quake hit central Mexico three days earlier. Hide Caption 1 of 42 Photos: Deadly earthquake rocks central Mexico Rescue workers search for survivors Thursday, September 21, at a collapsed apartment building in Mexico City. Hide Caption 2 of 42 Photos: Deadly earthquake rocks central Mexico A religious statue is salvaged from a former convent that was heavily damaged in Tlayacapan, Mexico. This was the second earthquake to hit Mexico in two weeks. A magnitude 8.1 quake struck off the country's southern coast on September 8. Hide Caption 3 of 42 Photos: Deadly earthquake rocks central Mexico A wall is damaged at a home in Tlayacapan on Wednesday, September 20. Hide Caption 4 of 42 Photos: Deadly earthquake rocks central Mexico Volunteers organize donations in Mexico City on September 20. Hide Caption 5 of 42 Photos: Deadly earthquake rocks central Mexico Rescuers in Mexico City work to save a child trapped inside the Enrique Rebsamen elementary school on September 20. Rescue workers said they believed they'd made contact with a girl trapped in the rubble at the school. But by the next afternoon, navy official Angel Enrique Sarmiento said all the school's children had been accounted for and there was no student in the rubble. He apologized for the confusion. Hide Caption 6 of 42 Photos: Deadly earthquake rocks central Mexico Men carry beams of wood to offer help in Mexico City's Roma neighborhood on September 20. Hide Caption 7 of 42 Photos: Deadly earthquake rocks central Mexico Families prepare to sleep under tarps outside their quake-damaged building in Mexico City on September 20. Hide Caption 8 of 42 Photos: Deadly earthquake rocks central Mexico Rescuers and firefighters lower a corpse from a house in Mexico City on September 20. Hide Caption 9 of 42 Photos: Deadly earthquake rocks central Mexico Volunteers and rescue workers search for people trapped inside the Enrique Rebsamen school on September 20. Hide Caption 10 of 42 Photos: Deadly earthquake rocks central Mexico A volunteer in Mexico City asks for silence as a flattened building is searched for survivors on September 20. Hide Caption 11 of 42 Photos: Deadly earthquake rocks central Mexico Members of the Mexican Army nap September 20 after assisting in search-and-rescue missions in Mexico City. Hide Caption 12 of 42 Photos: Deadly earthquake rocks central Mexico People look for survivors in Mexico City on September 20. Hide Caption 13 of 42 Photos: Deadly earthquake rocks central Mexico A man walks his bike past a partially collapsed building in Jojutla on September 20. Hide Caption 14 of 42 Photos: Deadly earthquake rocks central Mexico Volunteers arrange food and other donated supplies at a distribution point in Mexico City on September 20. Hide Caption 15 of 42 Photos: Deadly earthquake rocks central Mexico Children's toys are seen in a damaged building in Mexico City on September 20. Hide Caption 16 of 42 Photos: Deadly earthquake rocks central Mexico A survivor is pulled out of rubble in Mexico City on September 20. Hide Caption 17 of 42 Photos: Deadly earthquake rocks central Mexico People sleep on the street next to damaged homes in Jojutla on September 20. Hide Caption 18 of 42 Photos: Deadly earthquake rocks central Mexico Soldiers remove debris from a collapsed building in Mexico City on September 20. Hide Caption 19 of 42 Photos: Deadly earthquake rocks central Mexico An injured person is carried away after being rescued in Mexico City on Tuesday, September 19. The earthquake happened on the anniversary of a 1985 quake that killed an estimated 9,500 people in and around Mexico City. Hide Caption 20 of 42 Photos: Deadly earthquake rocks central Mexico Rescue workers remove rubble from a Mexico City building on September 19. Hide Caption 21 of 42 Photos: Deadly earthquake rocks central Mexico Rescue workers in Mexico City search for people trapped inside the collapsed Enrique Rebsamen school on September 19. Hide Caption 22 of 42 Photos: Deadly earthquake rocks central Mexico A man comforts a student outside a school in Mexico City on September 19. Hide Caption 23 of 42 Photos: Deadly earthquake rocks central Mexico A police officer runs toward the site where a building collapsed in Mexico City on September 19. Hide Caption 24 of 42 Photos: Deadly earthquake rocks central Mexico Rescue workers and volunteers search a collapsed building in Mexico City on September 19. Hide Caption 25 of 42 Photos: Deadly earthquake rocks central Mexico A man is rescued under rubble in Mexico City's Condesa area on September 19. Hide Caption 26 of 42 Photos: Deadly earthquake rocks central Mexico Hide Caption 27 of 42 Photos: Deadly earthquake rocks central Mexico A woman's crushed body hangs from a collapsed building in Mexico City on September 19. Hide Caption 28 of 42 Photos: Deadly earthquake rocks central Mexico Recovery efforts take place at the collapse of a residential building in Mexico City on September 19. Hide Caption 29 of 42 Photos: Deadly earthquake rocks central Mexico The quake damaged the Jojutla Municipal Palace. Hide Caption 30 of 42 Photos: Deadly earthquake rocks central Mexico A search goes on at the scene of a collapsed building in Mexico City's Del Valle neighborhood on September 19. Hide Caption 31 of 42 Photos: Deadly earthquake rocks central Mexico Emergency workers remove debris as they search for survivors in Mexico City on September 19. Hide Caption 32 of 42 Photos: Deadly earthquake rocks central Mexico People react in Mexico City just after the quake hit. Hide Caption 33 of 42 Photos: Deadly earthquake rocks central Mexico People gather on a Mexico City street after office buildings were evacuated because of the quake. Hide Caption 34 of 42 Photos: Deadly earthquake rocks central Mexico A building is damaged in Mexico City on September 19. Hide Caption 35 of 42 Photos: Deadly earthquake rocks central Mexico A woman in Mexico City cries as she tries to reach people on her cell phone after the quake. Hide Caption 36 of 42 Photos: Deadly earthquake rocks central Mexico Patients from a Mexico City hospital receive treatment outside after the hospital was evacuated on September 19. Hide Caption 37 of 42 Photos: Deadly earthquake rocks central Mexico A woman in Mexico City reacts after the quake. Hide Caption 38 of 42 Photos: Deadly earthquake rocks central Mexico People remove debris off a building that collapsed in Mexico City. Hide Caption 39 of 42 Photos: Deadly earthquake rocks central Mexico A car is crushed by debris in Mexico City on September 19. Hide Caption 40 of 42 Photos: Deadly earthquake rocks central Mexico A woman receives medical assistance after she was injured in Mexico City on September 19. Hide Caption 41 of 42 Photos: Deadly earthquake rocks central Mexico People stand inside a Mexico City building that collapsed in the quake. Hide Caption 42 of 42

Peña Nieto asked people to stay indoors and stay away from the streets while the rescue attempts continue. Still, residents are joining forces with rescue workers to search for survivors.

Mexico's soccer teams are helping in the relief effort, with C.F. Monterrey, Tiburones Rojos de Veracruz, C.F. Pachuca, Santos Laguna, Puebla F.C. and Club Atlas urging people to bring water and nonperishables to stadiums and other facilities.

In the upscale district of Roma, one of the hardest-hit areas of Mexico City, Violeta Gaytan was among those who fled to get away from damaged buildings and a strong smell of gas.

Temblor #cdmx #temblor #terremoto #eartquake A post shared by Don Jose Montero de T y Aragon (@virusventor) on Sep 19, 2017 at 12:31pm PDT

"There was a lot of people screaming, children everywhere. I saw people calling out for doctors and a lot of dust," the publicist said.

Like others, Gaytan began to walk home. She expected it would take hours, but a man was driving around offering people rides to metro stations across the city.

Hours before Tuesday's earthquake, authorities held a citywide drill on the anniversary of the 1985 earthquake that killed an estimated 9,500 people in and around Mexico City, one the country's worst natural disasters.

Less than two weeks ago, a magnitude-8.1 earthquake struck off Mexico's southern coast, killing at least 90 people, according to the governor of Oaxaca state.