BEIJING - Chinese Premier Wen Jiabao vowed Thursday to punish any corrupt person found responsible for a high-speed train crash that killed at least 39 people and triggered public anger over its handling.

Wen, speaking in a rare public news conference at the site of Saturday's crash between two bullet trains, said a "serious investigation" was under way and that results would be made public. The crash near the eastern city of Wenzhou also injured 190 people.

"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 while answering questions from Chinese and foreign reporters under the viaduct from which four train cars plunged about 65 to 100 feet (20 to 30 meters) after the crash.

"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," he said.

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.

In an apparent response to public criticism that it took him six days to visit the crash site, Wen explained that he had been ill in bed for the past 11 days. This was quickly refuted by users on China's popular Twitter-like site, Weibo, who pointed to news reports of him attending official meetings and state visits last week.

Wen's comments were made shortly after a railway official said design flaws in signal equipment and human error caused the crash in which six train cars derailed and four fell after one train plowed into the back of another that had stalled after being hit by lightning.

An Lusheng, head of the Shanghai Railway Bureau, also said that dispatchers did not send any warnings after the lightning strike.

"After the lightning strike caused a failure, an interval signal machine that should have shown a red light mistakenly upgraded it to a green light instead," An said in comments carried by state broadcaster CCTV.

The Beijing National Railway Research and Design Institute of Signal and Communication, which designed the signal equipment, on Thursday issued a letter of apology to the victims' families and the injured passengers.

It said it would cooperate with the investigation and would "have the courage to assume responsibility and accept the punishment deserved."

Despite the apology, many questions remain, in particular, why lightning was able to stop the train and why no other systems were in place to prevent such a crash.

The firing of three top officials at the Shanghai Railway Bureau has done little to tamp down criticism that authorities made only passing attempts to rescue survivors while ordering tracks swiftly cleared to restore service.

Wen acknowledged that the accident could hurt trust in China's high-speed railway technology in the country's export markets.

"If we lose safety, we lose the high-speed railway's credibility," he said. "In these years, we've made great achievements in the high-speed railways. But the accident reminds us that we should attach more importance to safety in high-speed railway construction."

During his visit, tens of relatives of people who died in the crash protested in front of the Wenzhou city government building, holding up a banner urging Wen to help uncover the "truth" behind the accident. Relatives also gathered Wednesday at the Wenzhou South Station, with pictures and signs demanding answers, Chinese media reported.

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