Chinese officials say a design flaw and human error combined to cause the nation's deadliest train accident since 2008.

State media Thursday quoted railway officials saying a severe flaw in the design of signaling equipment prevented a green light from turning to red after a lightning strike halted a train on a high-speed line south of Shanghai. That caused a second train to smash into the rear of the first, toppling several carriages off of a bridge.

The officials also said dispatchers at a station in Wenzhou should have intervened manually after the Chinese-made signaling equipment failed. The accident killed 39 people and injured almost 200.

Premier Wen Jiabao traveled Wednesday to Wenzhou to oversee an investigation of the accident and console the relatives of the victims. Chinese websites and newspapers have carried unusually harsh criticism of the nation's railway officials since the accident on Saturday.

More than 100 relatives of the victims gathered at the railway station in Wenzhou on Wednesday to demand an explanation for the accident. Stung by the criticism, the government has ordered a two-month safety campaign for the rail system and launched an intensive investigation. Three rail officials have already been fired.

Internet users exploded in outrage when it was revealed that authorities had buried some of the wreckage at the scene of the crash, rather than removing it for examination. Officials said it was done to facilitate rescue efforts.

There were also complaints that authorities were too quick to call off the search for survivors. A two-year-old girl was found alive hours after the search was ordered to be ended.