Six passengers boarding a Ural Airlines flight were injured after an airstair collapsed. The incident took place when the passengers were boarding Airbus A320 from the Siberian city of Barnaul to Moscow on Tuesday morning.

A video of the incident shows people falling onto the tarmac after the top of the airstairs collapsed as they were just about to board the flight.

Airport officials said three of the six injured passengers were rushed to a nearby hospital. While one of them is suspected to have a broken leg, the other may have suffered a back injury, the Sun reported.

“They all fell down at once, it was like watching a computer. I was about to step up the ladder when it happened, and also the first who ran to them to help,” a passenger said, the Siberian Times reported.

“One of the men broke his leg. Several other passengers got injured but it wasn’t immediately clear how bad it was. A young woman was lucky as she wasn’t injured at all, she just got really shaken. We all got quite a fright,” the passenger added.

An airport representative said the accident took place due to a “hidden fault” on the airstairs which was “impossible to detect” during the regular pre-flight check.

A Ural Airlines spokesperson, however, said the airline was not responsible for the accident as the airstair was the property of the airport. The airline was also not planning to compensate the injured passengers.  

Oksana Gorbunov, a Western Siberia transport prosecutor’s official, said investigation was ongoing to know what went wrong.

Another video released showed new steps being taken to the plane. The flight arrived at Moscow airport almost two hours late.