Federal investigators have finished collecting evidence at the site of a deadly gas pipeline blast in a San Francisco suburb and are now interviewing people who saw the massive fireball.

National Transportation Safety Board spokesman Peter Knudson says investigators still don't know what sparked Thursday's explosion. The fire killed at least four people and destroyed nearly 40 homes.

This week, the NTSB removed a segment of the pipe and shipped it to a lab in Washington, where it will be analyzed.

Knudson could not say how long it will take the agency to release the test results. They could provide the evidence needed to find out if corrosion or some other factor caused the gas line to rupture.

Meanwhile, investigators plan to stay in the area through the weekend to interview anyone who saw the explosion.