SAN BRUNO, Calif. – 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, and even as dozens of people returned to their scorched homes officials tried to confirm the death toll.

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 the blast were found Sunday, city spokeswoman Robyn Thaw said.

PG&E spokesman Andrew Souvall said Sunday that the ruptured section, which was installed in 1956, was last checked for leaks in March. The company said later Sunday no leaks were found.

Crews on Sunday packed into a crate the 28-foot section of ruptured natural gas pipeline blown out of the ground and hurled 100 feet in the explosion, National Transportation Safety Board vice chairman Christopher Hart said.

Investigators were to ship the pipeline to an NTSB metallurgy lab for examination, he said.

Also being shipped were two 10-foot sections of pipe removed from the crater Sunday from either side of where the ruptured section had been.