California ? Residents returned Sunday to the ruined hillsides of their suburban San Francisco neighborhood, three days after a natural gas pipeline exploded into a deadly fireball.

A nearby risky segment of the gas line was due to be replaced, the utility responsible said, because it ran through a heavily urbanized area and the likelihood of failure was “unacceptably high.” That 30-inch diameter pipe a few miles north was installed in 1948, and was slated to be swapped for new, smaller pipe.

California regulators ordered the utility, Pacific Gas and Electric, to survey all its natural gas lines in the state in hopes of heading off another disaster.

Investigators still don’t know what caused Thursday night’s blast. The remains of at least four people have been found, and authorities have said four are missing and at least 60 injured, some critically. Two people reported missing after blast were located Sunday, city spokeswoman Robyn Thaw said.