Emergency services on Tuesday worked to retrieve the bodies of victims from the wreckage of an elevated metro line that collapsed in the Mexican capital, leaving at least 23 dead and dozens injured.
President Andres Manuel Lopez Obrador promised a full investigation into Monday night's accident, which raised questions about construction standards on a network used by millions every day.
Carriages were seen hanging from the metro overpass in the south of the capital in a tangle of twisted cables with the ends pointing towards the ground in a V-shape.
Several minors were among the 23 dead, Mexico City Mayor Claudia Sheinbaum told reporters.
Four of the victims were still in the train, she added.
Nearly 80 others received hospital treatment following the dramatic accident, one of the worst to strike the Mexico City metro since it was inaugurated in 1969.
Earlier, anxious relatives had gathered at the site awaiting news of those believed to have been aboard the train.
Efrain Juarez said that his son was in the wreckage.
"My daughter-in-law called us. She was with him and she told us the structure fell down over them," he told AFP.
Another man who did not give his name said that his brother was among those trapped.
"He came with his wife and they managed to get her out, but he was crushed there and we don't know anything," he said.
'Structure was shaking'
Dozens of emergency workers were seen trying to rescue victims from the carriages.
The work had to be suspended because of fears the wreckage was too unstable, but it later resumed with the help of a crane.
A car was trapped under the rubble, but a person inside managed to get out alive, Sheinbaum said.
The police, who brought rescue dogs, urged onlookers to move back in case of falling debris.
"Suddenly I saw that the structure was shaking," an unidentified witness told the Mexican television network Televisa.
"When the dust cleared we ran... to see if we could help. There were no screams. I don't know if they were in shock," she added.
Medics were seen taking the injured away on stretchers.
The casualties were rushed to different hospitals in the city, Sheinbaum said.
She promised a structural examination of the affected metro line, which will remain closed, and a full investigation into the causes of the accident.
"We will report the whole truth. Our support to all victims," Sheinbaum tweeted.
The metro line that partially collapsed was built while Foreign Minister Marcelo Ebrard, a close ally of Lopez Obrador, was mayor of the capital from 2006-2012.
"A thorough investigation will be carried out ... to know the truth," the president said at his daily news conference, adding that independent international experts would be involved.
"We cannot get into speculation, much less blame the possible perpetrators without having proof. We will have the investigation carried out soon," Lopez Obrador said.
Ebrard said that it was "a very sad day for everyone" and he shared the outrage and pain over the tragedy.
"For my part, I put myself at the full disposal of the authorities," he said.
"I understand that there are many motivations of a political nature," added Ebrard, considered among the possible ruling party candidates for the 2024 presidential election.
One man, Jose Martinez, told reporters that he had a miraculous escape because he was unable to leave work in time to catch the ill-fated train.
"I was saved by like 15 minutes. It's good that nothing happened to me," he said.
Ricardo de la Torre, a Mexico City resident who lives close to Line 12, said that he had been worried about the quality of the overpass because the trains make nearby buildings shake.
"By that simple fact we know that the construction is bad," he said.
Monday's incident comes just over a year after two subway trains collided in Mexico City, leaving one dead and around 40 injured as panicked passengers escaped through dense smoke.
In January of this year, one person died and 29 suffered smoke inhalation injuries in a fire in the metro's control center.
In one of the worst ever accidents on the network, two metro trains rammed into each other leaving 23 dead and 55 injured in October 1975.