BEIJING — A truck that exploded and caused an elevated stretch of highway to collapse in central China, killing 10 people, was loaded with holiday fireworks that were illegally produced and transported, authorities said Saturday.

Local authorities have shut down the company that made the fireworks, Hongsheng Fireworks Manufacturing Co. Ltd., and detained four company officials after Friday’s blast, state media reported.

It remained unclear what set off the fireworks as they were shipped eastward on a major highway through Henan province. State-run China Central Television said witnesses thought a collision caused by heavy smog might have triggered the blast, which occurred about 55 miles west of the ancient city of Luoyang.

The Ministry of Public Security said Hongsheng, based in the neighboring province of Shaanxi, had illegally produced the explosives, packaged them in disguise and contracted with a trucking company unlicensed to handle hazardous commodities.

It said the factory had failed to check the credentials of the trucking company’s personnel. The state-run China News said the explosives had been declared as general commodities.

Preliminary investigations blamed the explosion for the collapse of the 260-foot stretch of the elevated highway in Mianchi county, sending trucks and sedan cars plummeting 79 feet to the ground, according to a statement by the provincial government of Henan.

Front Section, Pages 6 on 02/03/2013