The collapse occurred at around 10:30 p.m. local time on Line 12, the newest of Mexico City's subway lines, which runs underground through more central areas of Mexico's densely populated capital but then emerges onto elevated structures along the outskirts. A support beam "gave way" just as the train passed over it in the southern borough of Tlahuac, according to Mexico City Mayor Claudia Sheinbaum.
Footage from the scene showed a crane working to hold up one subway car left hanging from the collapsed section of the overpass, with cars buried under the rubble on the road about 16 feet below.
"We don’t know if they are alive," Sheinbaum said at a press conference early Tuesday, speaking about the people possibly trapped inside the train and cars.
Emergency crews worked through the night to remove people -- dead and alive -- from the scene. By early morning, authorities confirmed that there were no more bodies as they began to remove the wreckage.
At least 49 of the 65 people injured were transported to hospitals, including seven who were in serious condition and undergoing surgery, according to Sheinbaum.
"There are, unfortunately, children among the dead," the mayor told reporters, without specifying how many.
The Mexico City Attorney General’s Office has opened an investigation into the deadly incident.
Authorities will inspect the rest of Line 12 where the collapse happened later Tuesday morning. The subway line, which serves 20 stations, will remain closed in the meantime and hundreds of buses will be brought in to cover the service, according to Sheinbaum.
"This is an unfortunate and serious accident," she said. "We will report the truth."
Mexican Foreign Relations Secretary Marcelo Ebrard, who was Mexico City's mayor when Line 12 was built, called the incident "a terrible tragedy." Soon after Ebrard left office as mayor, the subway line became plagued by structural issues, technical faults and corruption allegations, leading to a partial closure in 2013 so tracks could be repaired.
"Of course, the causes should be investigated and those responsible should be identified," Ebrard wrote on Twitter. "I repeat that I am entirely at the disposition of authorities to contribute in whatever way is necessary."
ABC News' Will Gretsky, Joshua Hoyos and Joseph Simonetti contributed to this report.