Two men in Hong Kong have been arrested on suspicion of tampering with an escalator that had malfunctioned and injured 18 people, the BBC reports.

The harrowing video caught on witness cell phones shows an escalator packed with shoppers at a Langham Place mall. The escalator suddenly reversed, causing riders to lose their balance as a wave of victims piled on top of them.

According to CCTV, 18 people were injured in the incident.

According to the BBC, officials called in two technicians with the Otis Elevator company to investigate the incident. At the time, the escalator was turned off and those investigating were told not to touch it. However, it was later discovered the escalator’s auxiliary braking system had been reactivated.

The two employees were arrested. Otis told the BBC it is seeking clarification.

Officials speculate the escalator originally malfunctioned due to a damaged driving chain and a malfunction of the braking device.