Officials said on Friday that they believe a loose component on an advertising panel caused the incident, which led to serious disruption on the Island Line during the evening rush hour on Thursday.

They said the component in question had since been removed from eight advertising panels in Causeway Bay, Tsim Sha Tsui, Mong Kok, and Admiralty stations. Workers were to inspect and reinforce other advertising boards, they added.

Speaking to an RTHK radio program this morning, the railway company's chief of operating, Sammy Wong Kwan-wai, said an overnight investigation had ruled out a design flaw with the train door as the cause of the incident.

Wong said they are now focusing on the design and conditions of an advertising panel on the line and the procedures for changing advertising posters.

He said the last poster change was on Thursday last week. They will conduct a thorough investigation on how the component got loose and whether the incident was related to the operation and the panel's design and conditions.

Wong added that the trains had undergone strict inspections before going into service and believed only an enormous impact would have ripped off the carriage doors.

Meanwhile, a former council member of the Hong Kong Institution of Engineers, Henry Cheung, said he believed human error could be blamed for the incident.

"Is it possible the contractor wasn't thorough enough in meeting the MTR's safety requirements? It requires the MTR to investigate. The cause of the incident may involve human error," Cheung told the same radio program.

"It's possible that the component wasn't properly installed, and after a day of trains going past it and with vibrations, it became loose."

Cheung stressed the doors of the MTR trains are safe as the trains' carriage door design was adopted by many other train manufacturers around the world. However, he still suggested people not lean on them.