Missing screws under a platform of the moving walkway were the cause of the incident at Don Mueang Airport that cost a woman a part of her left leg on June 29, an observer in the fact-finding probe said on Friday.

Assoc Prof Weerachai Phut-dhawong, a lecturer at Kasetsart University, was appointed by the victim's family to join the fact-finding committee, which also comprises the Ministry of Transport, the Engineering Institute of Thailand, the Engineers Council and the Siam Hitachi Company.

The incident occurred last Thursday around 8.40am in the domestic terminal at the airport. The injured woman is being treated in an ICU room at Bumrungrad International Hospital.

Speaking after the committee meeting on Friday, Weerachai said the mechanical failure might have been caused by missing screws under the platform.

He said normally, each panel of the moving walkway has been tightened with four screws, but the panel that the woman stepped on had only one screw. Three screws were missing, so when she stepped on the panel, it fell while the walkway was still moving and trapped her leg.

"I view this as an accident, and it has nothing to do with luggage or the missing comb of the travellator. The travellator had been used for almost 30 years, and the sensor system was functional as it stopped the walkway in 10 seconds," he said.

However, the 10 seconds seemed to be too long as it appears to have cost the woman her leg, he said.

The question is how those screws went missing, he said, adding it might be because the moving walkway was too old.

Airports of Thailand is preparing to issue a statement admitting fault and taking full responsibility for the accident, according to the victim's son.