|Saturday's deadly train crash in eastern China left 39 passengers dead and about 200 injured [Reuters]
Railway authorities in China have said design flaws in signal equipment and human error are to blame for Saturday's deadly train crash in which 39 people were killed, according to the state news agency, Xinhua.
A Chinese railway research institute took responsibility on Thursday for a flaw in signalling equipment.
The Beijing National Railway Research and Design Institute of Signals and Communications Co Ltd, in a rare admission of responsibility for the disaster, issued an apology, acknowledging that it was the source of the deadly flaw.
The accident, in which nearly 200 people were injured, was China's worst rail accident since 2008, and has led to a public outcry.
The showcase high-speed line between Beijing and Shanghai has been plagued by power outages and other malfunctions since it opened on June 30.
More than 100 relatives of passengers who were killed protested on Wednesday outside a railway station, angered by the lack of accountability over the incident, state media reported.
The Global Times, a tabloid owned by Communist Party mouthpiece the People's Daily, said the protesters demanded direct talks with officials from the Railways Ministry.
Wen Jiabao, the Chinese premier, visited the crash site near Wenzhou city in China's eastern Zhejiang province and vowed a thorough and transparent investigation.
"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."
The accident was caused when a bullet train crashed into the back of another that had been stalled after being struck by lightning.
Following the accident, China dismissed three senior railway officials, including the head of the Shanghai railway bureau, his deputy and the bureau's Communist Party chief.
The three will "be subject to investigation", China's railways ministry said in a statement on its website.
Wang Yongping, the railways ministry spokesman, said: "As leaders ... they should take ultimate responsibility for the main cause of the accident."
Controversy, however, is not new to the burgeoning rail line.
Liu Zhijun, the railways minister, was dismissed this spring amid an investigation into unspecified corruption allegations.
No details have been released about the allegations against him, but news reports say they include kickbacks, bribes, illegal contracts and sexual liaisons.
China has spent billions of dollars and plans more massive spending to link the country with a high-speed rail network. Official plans call for China's rails to expand to 13,000km this year and 16,000km by 2020.
However, the accident has raised questions about the safety of the country's fast-growing rail network.
"There's been a lot of talk in the country, a lot of suspicion in China about the bullet trains and in many ways this actually really confirms a lot of people's suspicions about the safety and the technology and whether China is really ready to adopt this technology," Al Jazeera's Melissa Chan, reporting from the site of the accident in Wenzhou, said.