WALNUT CREEK (KRON) -- A natural gas smell that prompted evacuations and shelter orders in the East Bay has now dissipated but authorities still aren't sure what caused it.

The first reports of the smell came at about 8:00 a.m. Monday with calls about a rotten eggs odor in a six to eight block radius of Downtown Walnut Creek. Soon the smell had spread to Las Lomas High School where officials moved the students and staff to the football stadium. A shelter-in-place was put into effect at Pleasant Hill Elementary. Police and sheriff's deputies helped managed the evacuation of the courthouse on Ygnacio Valley Road.

PG&E workers armed with sophisticated detectors could be seen walking through the area. Officials got calls from as far away as Orinda and Alamo.

Investigators never did find a source for the gas smell. The speculate the odor might have come from a truck carrying natural gas or propane that had a leak as it drove down the road. That, they say, may explain why the smell was reported in so many different places.

The are no reports of serious injuries in connection with the smell.

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