A boiler pipeline went off in a plant in Shanghai Sunday morning, leaving at least three people injured and a nearby subway line off service for more than two hours.

A section of the boiler pipeline exploded at about 3:30 a.m. in Jianhua Concrete Pile Co. Ltd. in Songjiang District, southwest Shanghai, slightly injuring at least three people, according to officials with the Shanghai Work Safety Supervision Administration.

Part of the debris of the blast, which is estimated to weigh about 1 ton, hit some piers of the ground section of the No. 9 subway line built adjacent to the plant. It forced the postponement of the service scheduled to begin at 5:30 a.m to 8 a.m. for equipment examination and maintenance.

Local subway and traffic authorities mobilized many buses to help transport passengers until the subway operation resumed.

The cause of the blast is under investigation.