A four-month investigation launched after an escalator in a Beijing subway station suddenly went into reverse, resulting in one death and 30 injuries, has found the manufacturer, Otis, responsible for the tragedy. The investigation by the Beijing Bureau of Quality and Technical Supervision concluded the escalator's design was seriously flawed. It said the maximum load in motion was not verified when the original two-motor design was changed to a one-motor design, Xinhua reported, citing the findings of a bureau investigation panel. The report said a bolt broke, causing the drive chain in the escalator's motor to fall off, leading to the sudden reversal. An auxiliary brake also failed to function properly. A 13-year-old boy died and two of the 30 people hurt were said to be in a serious condition after the incident at Beijing Zoo Station on Subway Line Four on the morning of July 5. The line was built and is operated by a joint venture in which Hong Kong's MTR Corporation holds a 49 per cent stake. The probe team said Guangzhou Otis Elevator, which manufactured the escalator, and Beijing Otis Elevator, which provided maintenance services, should take major responsibility for the accident. It also said Otis Elevator (China) Investment should take secondary responsibility for not releasing relevant technical documents in time. Guangzhou Otis and Beijing Otis, both affiliated to the US-based escalator giant Otis, have been fined 200,000 yuan (HK$244,000) each. Otis China was fined 100,000 yuan. The investigation team said mechanics of Beijing Otis who were responsible for maintenance of the escalator would have their licences revoked. The team also said that two people responsible for the accident should be prosecuted. Beijing suspended the operation of 177 Otis escalators used in its subway stations. Otis is 'rectifying the escalators' and promised to have them reopened by Lunar New Year, it said. The General Administration of Quality Supervision, Inspection and Quarantine ordered a nationwide halt to the use of the same model of Otis escalator after the Beijing accident. The administration said an initial investigation showed that the accident was a result of design and machinery flaws in that model. Otis has been under fire after a string of accidents. Within days of the Beijing accident, an Otis lift in an office building in Shanghai, carrying more than a dozen people, fell suddenly before stopping. No casualties were reported and worn-out parts were blamed.