Twenty-four of the 30 people who were injured after an escalator at a Beijing subway stop malfunctioned Tuesday have been discharged from hospital, subway officials said Wednesday.

The 24 people who suffered slight injuries have been discharged, while the other six are still receiving medical treatment, according to Yang Ling, a spokeswoman with the Beijing MTR Corporation, which operates the subway line.

The injuries of the six people are not life-threatening, she added.

The escalator malfunction happened at 9:36 a.m. when the rising escalator suddenly changed its direction at the A exit of the Zoo Station on Line 4 subway, leaving a 13-year-old boy dead and 30 others injured.

The malfunction of a component led to the sudden loosening of the escalator's drive chain, according to a statement released by the municipal quality and technical supervision bureau Wednesday.

The bureau has urged subway and bus stations, airports and shopping malls to check escalators and automatic walkways to ensure their safety, said the statement.

The escalator, produced by the world's largest escalator producer OTIS, was still in the guarantee period and the manufacturer just completed a routine check on June 22, Yang said Tuesday.

Initial investigations show that the escalator's protection mechanism that was supposed to prevent such a sudden backward movement did not work, said a spokesman of Beijing Municipal Commission of Transport on Wednesday.

The spokesman said that a recall of OTIS's escalators may be needed if future investigations show flaws in the escalators' design or manufacture.

Before investigations results are released, Beijing's subway construction companies will not purchase any more OTIS's escalators, he said.

All the escalators in Beijing's subway stations had been checked as of Wednesday, he said.

Beijing has 126,000 elevators, including 14,000 escalators and automatic walkways.