MILL VALLEY, Calif. -- The head of the state Environmental Protection Agency Wednesday called for independent review of the regional water board after an investigation revealed a second major sewage spill into the San Francisco Bay.

State water officials said their investigation had revealed that last week's spill of 2.7-million gallons of partially treated sewage into the Bay was preceded by the release of about 2.5 million gallons of virtually raw sewage during a storm on Jan. 25.

The Sewerage Agency of Southern Marin reported the first spill to the state Regional Water Quality Control board, which oversees the water treatment plant in Mill Valley. But the Marin agency did not report to state officials how much sewage was released and used an incorrect date.

The regional water board was investigating both spills, but the Board's own actions may also be investigated.

Linda Adams, who heads the state Environmental Protection Agency, wants an independent review of the regional water board because it failed to immediately investigate the first sewage spill.

"Although it appears that the Sewerage Agency failed in its responsibility to report the original incident as required by state law, the Regional Board staff apparently missed at least two opportunities to investigate the matter," Adams wrote in a letter to Regional Water Board Executive Officer Bruce Wolfe.

"I respectfully request that you immediately authorize a thorough and independent investigation of the Regional Board's standard operating procedures concerning emergencies of this nature," the letter read.

Altogether, more than five million gallons of sewage was released into a creek that flows into Richardson Bay, an arm of San Francisco Bay between Sausalito and Tiburon.

The earlier release occurred during a storm that filled up a holding pond of sewage. Plant operators feared flooding of the Mill Valley plant and they released the sewage from the pond into adjacent Arroyo Corte Madera del Presidio Creek, said Lila Tang, chief of the wastewater division for the regional water board.

"They were concerned that if the pond spilled the plant would be flooded, and that would have presented even more problems with electronics," Tang told the Marin Independent Journal.

The Sewerage Agency of Southern Marin didn't notify the state Office of Emergency Services -- as required by law if a spill exceeds 1,000 gallons, Tang said.

Stephen Danehy, manager of the sewerage agency, declined comment when contacted at home by the newspaper.