SAN FRANCISCO (AP) -- Federal regulators investigating a natural gas explosion that damaged five San Francisco buildings will also look into how long it took to shut off the gas.

Investigators with the National Transportation Safety Board visited the scene Friday and took several feet of pipeline to examine in the laboratory.

NTSB board member Jennifer Homendy says there's no indication a crime was committed.

No one was injured Wednesday but flames shot above the rooftops of three-story buildings and burned for more than two hours until Pacific Gas & Electric workers shut off the gas.

The city fire chief has said a construction crew digging up a street cut the pipeline.

But officially determining the cause of the blast could take months or even years.