KUNMING - Substandard construction has been confirmed as the cause of Tuesday's fatal metro line accident in Kunming, capital of Southwest China's Yunnan province, said the metro operator on Friday.

Initial investigation showed that a fireproof door fell onto the track and caused a train to derail, killing one driver and injuring another, according to a report released by Kunming Rail Transit Co., Ltd.

Contractors failed to install the fireproof doors in accordance with construction specifications, said the report.

At 9:09 am on Tuesday, a test train on the metro line derailed, the ensuing collisions causing the vehicle's air conditioning device to fall and hit the drivers in their cabin.

The city's first metro line, scheduled for trial operation in February, was undergoing a non-loaded test run.

The government has set up an investigation team to probe the accident and has demanded the metro company start an overall safety check.

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