Authorities said that 33 people were injured in an accident involving two high-speed trains outside Philadelphia on Tuesday.

According to the Pennsylvania Transportation Authority spokesperson Heather Redfern, an inbound Norristown High Speed Line train crashed into an unoccupied, parked train at the 69th Street Terminal in Upper Darby.

The incident took place at around 12:15 a.m. local time.

The injured have been sent to various hospitals.

The cause of the incident is under investigation.