The sole rail investment and operation company in Chengdu reported on Wednesday that the city's subway system has used electric cables from a company that provided substandard products to the Xi'an metro system.

Three metro lines in Chengdu, capital of Sichuan province, including one already in operation, were found to have used the electric cables, according to Chengdu Rail Transit.

The two major contractors of Chengdu's metro system, China Railway Group Ltd and Power Construction Corporation of China, apologized to the public in a statement released on Wednesday for using the potentially dangerous cables and vowed to replace them all to reassure the public.

The substandard low-voltage cables found in Xi'an are reportedly susceptible to catching fire and the toxic gas released in case of a fire is lethal.

The provider of the cables was Shaanxi Aokai Cable Co, whose cables had been used on Line 3 of the metro system in Xi'an, capital of Shaanxi province, which transports an average of more than 340,000 people daily.

The Xi'an government announced at a news conference on Monday that all five samples of the cables provided by Aokai from subway Line 3 were substandard.

Eight people at the private company, established in late 2012,had been detained thereafter.

Local police also shut down the company's production sites, sealed its account books and took other cable products for further testing.

In another development, Hefei Urban Rail Transit Co, the subway operator in Hefei, capital of Anhui province, announced that it would launch an investigation into cables used in its rail transport projects, according to Xinhua.

Aokai reportedly sourced electric cables to the company in the second half of 2015, but a third-party's quality testing showed the cables met standards at that time.

Hefei Urban Rail Transit Co company said it is likely to publicize the results of its investigation on March 28.