An explanation by railway authorities for last Saturday's high-speed train crash has raised more doubts about what happened and about the government investigation.

A high-speed train rammed into another that was stopped near the city of Wenzhou in east China's Zhejiang Province because of "serious design flaws" in signaling equipment and human error, an official from the Shanghai Railway Bureau said on Thursday.

A lightning strike triggered the malfunction, which resulted in a green alert light failing to turn red, leaving railway staff unaware of the stalled train, the official said.

The Beijing National Railway Research and Design Institute of Signal and Communication Co (CRSCD), which had designed and built the signaling system, posted an apology on its website, offering condolences and promising to "shoulder any due punishments that may result from the investigation."

However, the explanation has done little to alleviate public concern.

In a report on the accident yesterday, the Beijing Youth Daily newspaper posed the following questions:

"Why was such seriously flawed equipment in use for nearly two years without being detected?

"Why was it installed in as many as 76 rail stations across the country?

"Are there other problems with the railway apart from equipment flaws?"

At a CRSCD press conference on Thursday afternoon, many questions posed by journalists were left unanswered. Tian Zhenhui, a company official, would not admit direct responsibility for the accident. Pressed by reporters, she said the company apologized only because it had helped to build the railway.

She refused to specify which pieces of equipment failed, what other lines the equipment was installed on and how the equipment managed to pass tests and quality checks before being installed.

"Why haven't you suspended the operation of railways that have installed similar equipment? What if more accidents occur while the investigation is under way?" asked one reporter.

"It's a technical issue. I have no idea about that," Tian repeatedly replied, stressing that there had yet to be an official conclusion to the investigation.

"Premier Wen Jiabao has demanded that the probe into the accident be open, transparent and subject to public oversight, but the first person to apologize for the accident cannot even explain what responsibilities she has," the China Youth Daily said.

Online, microblogs and message boards were full of posts expressing concern about the accident.

"Mere signal failures cannot cause such accidents on modern high-speed railways. There must be more inside stories," one Internet user posted after the press conference.

Zhang Quanling, a news anchor with China Central Television, took railway authorities to task with a post forwarded more than 24,500 times, asking: "Where else is the flawed equipment being used? Were the flaws checked and rectified by a third party before the railway resumed operation?"