Authorities arrested two mechanics after a crowded escalator in a Hong Kong shopping mall reversed direction without warning over the weekend, injuring 18 people.

Footage shows the 150-foot escalator operating normally, then suddenly reversing direction at high speed, sending dozens of people toppling over each other and tumbling toward the base.

“I heard people screaming,” one witness told the South China Morning Post. “The escalator was going down but the speed accelerated.”

The People’s Daily tweeted video of the escalator reversing on Saturday:

Escalator accident in #HongKong's popular mall Langham Place injured at least 18 on Saturday pic.twitter.com/NDMBi9kVMK — People's Daily,China (@PDChina) March 25, 2017

The escalator is located inside the Langham Place shopping center. It passed inspection last week, according to The Guardian.

Images tweeted by the South China Morning Post show emergency workers responding to the scene.

Escalator was going "twice as fast": witnesses describe accident at #HongKong shopping mall that left 17 injured https://t.co/CKAqnN76pE pic.twitter.com/qTFC6TW0z1 — SCMP News (@SCMP_News) March 25, 2017

Two mechanics, both employees of the Otis Elevator Company, were arrested in connection with the incident, CNN reported. They each face charges of obstruction of justice, police said.

“The arrest of our mechanics was a surprise,” Ian Fok, a spokesman for the Otis Elevator Company, said in a statement, according to the network. The suspects are 22 and 52 yeas old.

Initial reports suggested that a chain and emergency break malfunctioned on the machine.