President Barack Obama bowed his head during the moment of silence to remember those lost on Sept. 11, 2001.
President Barack Obama has called California Gov. Arnold Schwarzenegger to offer the federal government's help in the wake of the pipeline explosion that killed at least four people.
The White House says Obama spoke with Schwarzenegger on Saturday to express condolences over the loss of life and assure him resources would be available if needed to recover from the disaster.
Officials were still trying to determine what led up to the Thursday blast that killed at least four people and injured dozens of others in the hills overlooking San Francisco Bay.