Investigators have found evidence of corrosion on a natural gas pipeline that exploded in a massive fireball in Pennsylvania last week, scorching trees a quarter-mile away, damaging homes and burning a fleeing homeowner, the federal pipeline safety agency said Wednesday.

The cause of Friday’s blast remains unknown, but the corrosion indicates a “possible flaw” in the coating material applied to a pair of welded joints, one at the point of failure and another excavated after the explosion, the Pipeline and Hazardous Materials Safety Administration said in a corrective order issued to the pipeline operator.

Houston-based Spectra Energy said a 2012 pipeline inspection did not reveal any sections that required repair. Spectra and federal officials did not address questions about the safety of other sections of the pipeline in light of the possibly defective coating.

The coating material, which is supposed to protect pipelines from corrosion, was applied when the 30-inch pipeline was installed in 1981 “following construction welding procedures in the field at the time,” the agency said.

Decades ago, that coating was an “accepted and preferred method of coating” the pipeline weld joints, Spectra spokesman Creighton Welch said in an e-mail.

The company conducts a high-tech pipeline inspection every seven years, as well as routine field surveys of corrosion prevention systems, he added.

The explosion in Salem Township, Westmoreland County, destroyed one home, damaged three others and prompted evacuations.

The owner of the leveled home, 26-year-old James Baker, is hospitalized with burns to more than 75 percent of his body.