Design flaws in signal equipment and human error caused last weekend's high-speed train crash that killed at least 39 people, state media reported Thursday.
The reports come in the face of public anger in China about the government's handling of the accident near Wenzhou in Zhejiang province.
Premier Wen Jiabao has called for a sweeping and transparent probe into the crash between two bullet trains that also hurt more than 190 people.
"Design flaws were found in the signal light equipment at the Wenzhou South Station ... and the dispatchers did not send any warnings. And that's what caused the train crash," said an online transcript of a report by China Central Television, the state broadcaster.
Wen arrived at Wenzhou on Thursday to check on the investigation and the conditions of the survivors.
S peaking to reporters at the site of the crash, he vowed to punish anyone involved if there was corruption that caused the crash.
He said a "serious investigation" was underway and that results would be made public.
"No matter if it was a mechanical fault, a management problem, or a manufacturing problem, we must get to the bottom of this," Wen said. "If corruption was found behind this, we must handle it according to law and will not be soft. Only in this way can we be fair to those who have died."
Six train cars derailed and four fell about 20 to 30 metres from a viaduct Saturday night after one train plowed into the back of a stalled train.
The government has ordered a two-month safety campaign for its railway system amid questions about how the crash occurred. Wen called for the campaign to be widened to target all transport infrastructure, coal mines, construction sites, and industries dealing with dangerous chemicals.
Accidents prompts outpouring of anger
The accident was the biggest blow yet to China's burgeoning high-speed rail ambitions that have been highlighted as a symbol of the country's rising economic and technological prowess.
Rapid expansion of the services has been dogged by concerns about safety, corruption scandals and criticism that schedules are impractical and tickets too expensive for ordinary Chinese.
Open just one month, the much-ballyhooed 1,318-kilometre Beijing-Shanghai line has been plagued by power outages and other malfunctions.
Saturday's accident outside the eastern city of Wenzhou prompted an outpouring of anger among the public and even in the usually docile state media, with questions posed over the cause of the crash and the government's handling of the aftermath.
The firing of three top officials at the Shanghai Railway Bureau did little to tamp down criticism that authorities made only passing attempts to rescue survivors while ordering tracks swiftly cleared to restore service.