Authorities in northwest China's Shaanxi Province are questioning an advertisement company after its decorative balloons filled with hydrogen exploded and injured 12 people.

The balloons blew up Sunday morning as about 20,000 students, teachers and alumni gathered at Weinan Teachers University to celebrate the 50th anniversary of the university's founding, a spokesman with the Weinan city government said.

A witness surnamed Sun said he saw the balloons explode and catch fire mid-air, injuring at least a dozen people on the face, neck and arms.

A spokesman with the local meteorological bureau, the body that supervises the use of meteorological and advertising balloons, said the company's use of balloons had not been approved.

The bureau, which helped investigate the accident, suspects the balloons exploded as they were released from inside a sack.

"The sack had a small opening and the balloons rubbed against each other. It was static electricity that caused the balloons to explode," the bureau said in a report.

The bureau has announced a ban on hydrogen-filled balloons in the city in the wake of the accident.

The Ministry of Education banned the balloons' use in school activities in 2007 after an explosion injured nearly 100 students at a school in northwest China's Gansu Province.