MEXICO CITY (AP) — A two-train subway collision in Mexico City killed a male passenger, injured 41 people and disrupted service Wednesday on the bustling metro system serving this megalopolis of over 20 million people.

Mayor Claudia Sheinbaum said via Twitter that one of the trains apparently reversed into the other by accident the previous night, shortly before midnight.

Twenty-five of the injured were treated at the scene, and the other 16 were taken to hospitals, Sheinbaum said. All the injuries were “light to medium” and not life-threatening. Hours later, Mexico City prosecutor Rosa Icela Rodríguez said that only four of the 16 remained in hospitals.

She said the cause of the crash was under investigation.

Metro authorities said the two drivers of the trains were among those hurt.

Mexico City Metro director Florencia Serranía said at a news conference that the “black boxes" from both trains, which will provide a “second-by-second” record of what happened, were turned over to the city prosecutor's office and their information appeared to be intact.

Workers had separated the stacked metro cars and were working to clear the track. She said she expected the line to be ready for service Thursday morning.

Serranía said the accident occurred 20 minutes before the end of service Tuesday night when one train was headed to the garage to prepare for Wednesday morning service.

She added that an international expert had been hired to conduct an independent review of the incident.

Images of the accident published in local media showed wrecked subway cars derailed in the Tacubaya underground station, and rescuers carrying people away.

The Mexico City Metro system, one of the world's largest and most transited, has seen at least two serious accidents since it opened five decades ago.

In 2015 a train failed to brake in time and smashed into another at the Oceania station in the city's north, injuring 12 people. Authorities later blamed “double human error.”

In the most serious incident, two trains collided at the Viaducto station in 1975, killing at least 31 and injuring more than 70, according to the national newspaper El Universal.

Tacubaya is a key station for the Metro system, with three of its 12 lines intersecting there, and there were disruptions during the Wednesday morning commute.

Metro authorities said service on Line 1 would be reduced throughout the day with Tacubaya and a neighboring station out of action and 45 buses deployed to bridge the gap of about 2.5 miles (4 kilometers).

Serranía told Milenio television that about five cars on each train were damaged.

Mexico City's Metro system transported more than 1.6 billion passengers in 2018, according to official figures, or about 4.4 million per day.

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Associated Press writers Christopher Sherman and Mark Stevenson contributed to this report.