KAOHSIUNG, Taiwan — At least 25 people were killed and 267 injured when underground gas explosions ripped through Taiwan’s second-largest city, hurling concrete and cars through the air and blasting long trenches in the streets, authorities said Friday, as they searched for the cause.

The series of five explosions about midnight Thursday and early Friday struck a densely populated district where petrochemical companies operates pipelines alongside the sewer system in Kaohsiung, a southwestern port with 2.8 million people.

Firefighters called to the neighborhood in the late evening to investigate a gas leak were among the victims when the blasts went off hours later, upending at least six fire trucks in the rubble of pavement and dirt.