Federal investigators today finished their initial investigation of the site of Thursday's gas pipe explosion in San Bruno and are now focusing on reviewing documents and interviewing witnesses, a spokesman said.

The National Transportation Safety Board finished examining pipes that were laid under the gas line that exploded and concluded they did not contribute to the blast, board vice chairman Christopher Hart said.

It was initially unclear whether the excavation and installation of those pipes damaged the gas line, he said.

Investigators have now turned the ditch over to the city of San Bruno and PG&E so infrastructure can be restored, Hart said.

Transportation safety board investigators are now looking at documents from PG&E, visiting control and monitoring sites, and examining valves that have been preserved for evidence, Hart said.

The transportation safety board is also looking at seismic records to see if any activity has occurred recently that could have affected the pipes, Hart said.

The agency has also begun to receive results of toxicology tests conducted on PG&E employees. So far the workers have tested negative for drugs and alcohol, Hart said.

"We'll be interviewing witnesses, including people from PG&E, over the next few days," he said.