XI'AN -- Four people are under investigation for suspected "serious disciplinary violations" following the discovery of substandard subway cables in Xi'an, capital of Northwest China's Shaanxi province, the municipal discipline inspection authorities said on Tuesday.

The four include Zhang Shaobing, director of a construction division under the Xi'an Metro Company Limited, two others with the Xi'an Supervision and Inspection Institute of Product Quality, and one with the Xi'an Construction Safety Supervision Station, according to a statement on the official website of the Xi'an municipal commission for discipline inspection of the Communist Party of China.

A story posted on the Internet claimed subway Line 3 in Xi'an used substandard cables and had safety risks.

Official investigation confirmed the use of problematic cables.

Six quality officials were put under investigation over alleged negligence of duty, the commission said on Thursday.