An elevated section of Mexico City’s subway system collapsed on Monday night as a train made its way across, plunging several cars onto the road below and killing at least 25 people and injuring more than 70. An investigation is under way into the cause of the collapse.

What happened?

One of the support beams on an elevated track on Mexico City’s subway system gave way, causing several train cars to fall to the road below. As of Wednesday morning, 25 people had died and 38 people remained hospitalized, city authorities said.

The collapse took place at 10:30 p.m. local time, when relatively few people were on the train. The death toll could have been a lot higher earlier in the day, when the trains are full. Line 12 currently carries around 220,000 passengers a day.

What do we know about the victims?

Line 12 of the subway runs east-west across densely populated low-income areas of the southeast of the capital, connecting with several other lines. Most of the victims were likely working-class Mexicans on their way home from work.

How big is Mexico City’s subway system?

The Mexico City Metro system is one of the world’s busiest. It has 12 lines covering 141 miles, with 195 stations, 115 of which are underground, 55 overground and 25 elevated. On average it transports 4.5 million people a day.