Late Sunday evening, half of New York City’s subway system shut down for five hours due to a power surge, stranding hundreds of passengers in the process. New York Governor Kathy Hochul called it an “unprecedented system breakdown” of the Metropolitan Transportation Authority.

The Washington Post reports that the problems began when a power line went down. This triggered a voltage dip across the city, and two power plants and generators also went offline, forcing backup systems to come online. When trying to restore service, a power surge occurred. When this happened, the subway command center lost communications with 83 trains. The public address system also went down. Five trains with more than 550 people were stranded.

Part of the problem was the passengers themselves. While emergency personnel did evacuate as many people as they could, riders on two trains decided to leave cars on their own, a very dangerous act because tracks could be re-energized if a train was coming. Firefighters had to search the tracks to make sure there were no more remaining passengers before the subway could fully restore power.

After a phased “rolling restoration,” the system was finally back online at 1:30AM Monday morning.

Hochul has since ordered a deeper investigation into what exactly caused the surge as well as the other problems. So far, investigators have said there is no suspicion of terrorism or malicious actions.