The Chinese firm manufacturing SEPTA’s new rail cars is among the high-profile companies that use mica obtained with child labor, an NBC News investigation has found.

Jia Bo, president of China Railway Rolling Stock Corp.'s American branch, CRRC MA, said in a statement that he was not aware of the mining process and called for an investigation of the company’s supplier base. A review of suppliers began after NBC interviewed CRRC officials on Oct. 28, the company reported.

The company did not say which components of the rail cars require mica, a mineral.

SEPTA is asking its own questions about the use of mica mined by children, the agency said in a statement Thursday.

“SEPTA has requested more information from CRRC MA about its supply chain, and the process by which CRRC MA procures materials and components related to the manufacture of SEPTA’s multi-level rail cars,” the statement said. "SEPTA is also reviewing the contract to determine whether it has control over processes related to the supply chain.”

NBC’s report followed the trail of mica — used in products ranging from electronics to steel composites — from mines in Madagascar to products put out by Panasonic, Electrolock, and CRRC. Children as young as 4 work in the mines, NBC reported. They lack access to clean water, health care, and education, and suffer from medical conditions like headaches and back pain due to dangerous and grueling mining, pulling the mineral from the earth by hand.

The Dutch child protection group Terre des Hommes estimated that about 10,000 children work in mica mining in Madagascar, which has a population of about 26 million. The island nation, east of Mozambique, is one of the world’s poorest countries.

CRRC MA has begun providing rail cars to the Massachusetts Bay Transportation Authority. The Boston-based transit agency is reviewing its contract to determine whether it can exert any control over CRRC’s supply chain, NBC’s Boston affiliate reported.

The news is the latest blow to CRRC as it works to enter the American railroad market. The company is a dominant force worldwide, but CRRC MA, based in Springfield, Mass., only recently began contracting with American transit agencies. Its Springfield plant was completed two years ago.

The shells of CRRC’s American rail cars are built in China, then shipped to the United States to have their components installed.

The company scored high-profile wins with the Boston and Philadelphia contracts, along with Los Angeles and Chicago, and gained a reputation for significantly underbidding competitors. SEPTA ordered 45 multilevel cars for Regional Rail for $138 million, about $34 million less than the nearest bidder. Delivery of the cars is scheduled to begin next year.

The low bids, though, attracted attention from Congress, which has expressed concern that CRRC is owned by the Chinese government. Legislators feared trains made by a company with direct ties to a rival nation could be a security risk. They also have said CRRC was able to underbid its competitors because China was underwriting production costs. CRRC has denied receiving support from the Chinese government.

In September, the House passed a military spending bill that makes it significantly harder for CRRC to do business in the U.S. The House passed legislation to bar federal funding for public rail car purchases from Chinese state-owned enterprises. The bill awaits action in the Senate, but if it passes, it would make it impossible for CRRC to operate in America.