Hong Kong (CNN) Two mechanics have been arrested after an escalator packed with people in a Hong Kong shopping mall suddenly went into reverse.

Police say the suspects may have tampered with the machine's parts after the Saturday incident, which sent shoppers tumbling.

Eighteen people were injured when the escalator at Mong Kok's Langham Place switched direction from up to down. One man suffered a head injury and was in serious condition, according to CNN affiliate CCTV.

The mechanics, aged 22 and 52, are facing charges of obstruction of justice, police said. Both have been released on bail and must report back to police in mid-April.

"The arrest of our mechanics was a surprise," Ian Fok, a spokesman for the Otis Elevator Company, which employs the two men, said in a statement.

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