Serious flaws in a signalling system caused a fatal collision on China's high-speed rail network, officials say.
Thirty-nine people died when a train ran into the back of another which had stalled on a viaduct near Wenzhou after lightning cut its power supply.
The system "failed to turn the green light into red", said An Lusheng, head of the Shanghai Railway Bureau.
Chinese Premier Wen Jiabao, who has been visiting the crash site, vowed to "severely punish" those responsible.
"The country's development is for the people, so the most important thing is people's lives," Mr Wen told reporters at the scene.
"No matter if it was a mechanical fault, a management problem, or a manufacturing problem, we must get to the bottom of this.
"If corruption was found behind this, we must handle it according to law and will not be soft."
Mr Wen earlier promised to take steps to improve safety on the high-speed rail network - one of the government's flagship projects which it hopes highlights China's development.
Six carriages derailed and four fell between 20m to 30m (65ft to 100ft) from the viaduct after Saturday night's crash, which injured nearly 200 people.
The accident came just four years after the country's first high-speed trains began operating.
Rail experts had warned against the rush to build the world's longest and fastest high-speed rail network in record time amid safety concerns.
There are allegations that corners were cut during construction because of corruption, raising questions about infrastructure across the country.
The BBC's Martin Patience in Wenzhou says it is difficult to get to the truth because of a lack of transparency and accountability.
There is a real sense that things are perhaps being built too quickly and that safety is being jeopardised in the process, our correspondent says.
'Public relations disaster'
Mr Wen's visit to the crash scene comes amid growing public outrage at the accident.
Internet users and relatives of the victims have been angered by the government's apparent unwillingness to answer questions about the crash.
This has led to accusations of government "arrogance", amid suspicions of a cover-up.
The authorities have moved quickly to stem media coverage, urging reporters to focus on "extremely moving" stories, saying the overall theme should be "great love in the face of great disaster".
Chinese media have been ordered not to question the official line on the accident, but several newspapers have published editorials criticising the railway ministry.
In an unusually scathing editorial published in both its English and Chinese versions, the state-run Global Times on Wednesday said the government's handling of the accident aftermath was a "public relations disaster".
"The relationship between the government and the public is like that of a ship and water. Water can keep the ship afloat or sink it," it said.
Some relatives of victims, who include two Americans and an Italian, have reportedly refused compensation and instead demanded to be given answers.
The accident is seen as a blow to China's hopes of selling trains abroad in a bid to become a high-tech exporter.
Shares in Chinese rail and train builders have fallen sharply since the crash.