Investigators focusing on California pipeline; surveillance video shows explosion.
Investigators are still looking for four people missing since a massive explosion rocked the San Francisco suburb of San Bruno on Thursday.
The inferno destroyed close to 50 homes and killed at least four people.
Investigators are still looking for clues as to what caused the rupture of the natural gas pipeline that led to the disaster.
"Our first goal is to improve safety. If we see issues with safety, we'll put out recommendations immediately," says NTSB inspector Christopher Hart.
Part of the investigation will include analyzing surveillance video that caught the blast that set off the massive fire.
The pipe, ground zero for the explosion, was more than 50 years old.
The California Public Utilities Commission has ordered Pacific Gas and Electric to inspect all its natural gas lines statewide.
Most of the residents are now back in their homes after being allowed to return Sunday.
A Congressional hearing into the blast is expected this week.
The NTSB has sent parts of the damaged pipe to Washington for further analysis.