Mexico City - Mexico waited anxiously on Thursday for signs of life in the rubble of collapsed buildings as a desperate search for survivors of a devastating earthquake entered a third day.Authorities put the death toll from Tuesday's 7.1 magnitude quake at 272 people, with the number expected to rise. Volunteer rescue workers who have been seeking survivors since Tuesday fought off growing fatigue to continue removing tons of rubble at dozens of flattened buildings in the capital and across several central states.But time is running out. Experts say the average survival time in such conditions and depending on injuries is 72 hours.About 200 people were still reported missing in Mexico City and feared trapped in the rubble. People embrace during an outdoor Catholic Mass near the Enrique Rebsamen school. (Rebecca Blackwell, AP)In the central neighbourhood of Roma, rescue workers scrambled to locate 23 people believed to be in the wreckage of a collapsed seven-storey office building.They have already pulled 28 survivors from the mountain of rubble. No deaths have been reported at the site so far.Relatives of the missing waited in anguish for news.Aaron Flores's sister Karen and friend Paulino Estrada were both trapped inside.Estrada has managed to contact his family by cell phone, even making a video call. But there has been no news from Karen Flores. A rescue dog is helped by volunteers after he became exhausted during the search. (Eduardo Verdugo, AP)"We're feeling disoriented and desperate because we haven't heard anything from her," said her brother, 30.Phantom schoolgirl Authorities put the overall death toll at 137 people in Mexico City, 73 in Morelos state, 43 in Puebla, 13 in Mexico state, five in Guerrero and one in Oaxaca.However, the situation on the ground was chaotic, and the figures were rising - and sometimes falling.Highlighting the confusion that still reigned two days after the quake, one story that gripped the world's attention turned out to be false: that of a girl supposedly trapped alive beneath the rubble of a school that collapsed in Mexico City.Authorities denied on Thursday that the girl existed."We have carried out a full count with the directors of the school and we are sure that all the children are either safe at home, in the hospital or unfortunately died," Angel Sarmiento, a top officer in the Mexican marines, told journalists at the ruins of the Enrique Rebsamen school on the capital's south side."There are indications there may be an [adult] still alive in the rubble. There are traces of blood in the photographs, as if the person had dragged him or herself and may still be alive." The recovery effort so massive some donation centres are turning away volunteers. (Marco Ugarte, AP)Rescue workers had told journalists they were certain a girl was trapped beneath the rubble, but the different versions of the story varied widely.The story had made headlines around the world after the quake, injecting a ray of hope into a tragedy that killed 19 children and six adults at the school.Not giving up But real stories of hope continued to emerge from ruined buildings across the city, where more than 10 000 people lost their lives in a devastating earthquake in 1985. In the north of the city, a man who had been trapped for 26 hours and a 90-year-old woman were pulled alive from the rubble. President Enrique Pena Nieto said authorities were not giving up the search."The rescue and support effort in the buildings that collapsed is still on. We are not suspending it. We have to keep up the rescue effort to keep finding survivors in the rubble," he said during a visit to the state of Puebla, where the epicenter was.More than 50 people had been rescued from collapsed buildings in the capital as of late Wednesday, he said.US President Donald Trump called Pena Nieto and offered assistance. A US search-and-rescue team was deployed to Mexico, the American embassy said. Rescue teams have also flown in from Israel, El Salvador and Panama and more were expected from Costa Rica, Ecuador, Honduras, Colombia and Spain.Rescuers said the Israeli teams came with equipment enabling them to detect cellphone signals in the rubble.Tuesday's tragedy struck just two hours after Mexico held a national earthquake drill, as it does every year on the 1985 anniversary.A system of quake sensors was set up in 1993 along the Pacific coast, where tremors are more common.People in Mexico City were not warned by the system on Tuesday because the epicentre was only 120km outside the capital and thus outside the main area of sensor coverage, said Carlos Valdes of the National Centre for Disaster Prevention.Adding to the national sense of vulnerability, the earthquake struck just 12 days after another quake that killed nearly 100 people in southern Mexico.