Jesus Esteva, head of Mexico City's public works department, says the probe by Norwegian firm DNV observed deficiencies in building materials used including bolts and deformation of structural supports in the part of the line that collapsed.

The initial report by DNV, an external auditor, found "six deficiencies in the construction process" that helped to bring about the May 3 accident.

"The aforementioned allows us to submit, on a preliminary basis, that the incident was sparked by a structural fault," Esteva told a news conference on Wednesday. Officials did not permit questions from reporters.

Further reports on the accident were due in the coming months, Mexico City Mayor Claudia Sheinbaum said.

Deficiencies identified included the welding of bolts and their attachment to girders, missing bolts on some girders, different types of concrete used and unfinished or poorly executed welding, the city said.

The collapse, Mexico's biggest train accident in years, killed 26 people and put pressure on close allies of President Andres Manuel Lopez Obrador, as well as Mexican billionaire Carlos Slim.

The incident put a spotlight on Slim's construction business, Grupo Carso, which was involved in building the section of the Line 12 metro route that collapsed.

Grupo Carso said it would not comment until the final report into the accident was published.

Australian Associated Press