SAN BRUNO, Calif. (AP) — Records show utility crews replaced five miles of aging gas pipes under a Northern California neighborhood less than two decades ago but stopped just short of a segment that ruptured and exploded last year.

Documents released this week in a federal investigation into the deadly explosion show Pacific Gas & Electric Co. had replaced the pipes in 1994 for seismic safety concerns. The new, stronger steel pipes, however, ended about 300 yards from the epicenter of September’s blast.

PG&E spokesman Joe Molica confirmed to the San Jose Mercury News that the seismic upgrades were done. But he says officials are still working to determine why the work didn’t replace all the pipes in the area.

The explosion and resulting fire killed eight people and destroyed dozens of homes.