WENZHOU, Zhejiang - Premier Wen Jiabao vowed on Thursday to severely punish any corrupt individuals found responsible for Saturday's high-speed train crash that killed at least 39 people and triggered public fury.

Wen, speaking at a public news conference at the site of the tragedy, said a "serious investigation" was under way and that the results would be made public.

Officials with the Shanghai Railway Bureau admitted on Thursday that "design flaws" in signaling equipment were to blame for the crash that also injured 192 people.

Bureau chief An Lusheng said that the signal at Wenzhou South Railway Station should have switched to red after a lightning strike caused a train to stall on a viaduct but remained on green and rail staff failed to notice.

"If corruption is 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," Wen told reporters after laying flowers at the accident site near Wenzhou.

He explained that the reason he had not visited earlier was that he had spent 11 days in bed due to ill health. He met victims' families before the news conference and visited Xiang Weiyi, a 2-year-old survivor, in hospital who was found after rescue teams were told to suspend their search.

"The construction of high-speed railways should integrate speed, quality, efficiency and safety. And safety should be the major priority," Wen, who has ordered an "open and transparent" probe into the accident, said.

He also said that the State Council, or Cabinet, has set up a separate investigation team, consisting of several supervisory departments. "We must get a truthful conclusion that can endure the test of history," he said.

The accident was the biggest blow yet to China's burgeoning high-speed rail ambitions.

Victims' families accused rail authorities of not conducting adequate checks of the site before suspending rescue efforts. Families also said that they had been forced to agree to the cremation of their relatives before they could pick up compensation.

However, the arrival of the premier gave them encouragement, families said on Thursday.

"It will help to know the true cause of the accident," said the brother of Lin Xiao, who perished in the crash. He demanded that authorities disclose more information, such as where victims were seated on the train.

Wen's comments came shortly after a railway official said design flaws in signaling equipment caused the crash. Six carriages derailed and four of them fell off a 15-meter-high viaduct after a train plowed into the back of another that had stalled after being hit by lightning.

The Beijing National Railway Research & Design Institute of Signal and Communication was found to be responsible for the flawed signaling equipment. The company, under the State-owned China Railway Signal & Communication Corp, issued an apology on its website on Thursday, acknowledging responsibility.

The institute said in a statement that it would "face up to shouldering responsibility, and accept any punishment that is due, and will pursue those who were culpable".

Rail staff at Wenzhou South Railway Station were also found to have failed to notice the faulty signal.

"The accident reflected that the railway department's safety management" was not up to standard, Shanghai Railway Bureau chief An said.

Luo Lin, head of the State Administration of Work Safety and the investigation team leader, said that the probe into the crash will continue and they will give a more detailed report in mid-September.

After the investigation team made public their initial conclusions, many still felt that they needed to know more details.

"The signaling equipment has been used for nearly two years," Zhao Jian, professor at Beijing Jiaotong University, said. "Why did it not go wrong until now? The investigators may need more time to find out and answer these questions in detail."