As much as a million gallons of sewage has poured into Marin County's Corte Madera Creek this week - including sludge from a pipe that ruptured Wednesday - following a series of mishaps that officials believe could be sabotage.

The raw sewage bubbled out of a badly clogged pipeline in Kentfield Friday. Then, about 9:30 a.m. Wednesday, a large pipe apparently burst underneath a berm used by locals as a jogging trail next to the creek, about three-quarters of a mile west of Sir Francis Drake Boulevard.

An undetermined amount of effluent spewed out of the ruptured pipe, said Brett Richards, general manager of the Ross Valley Sanitary District.

"It was a major pipe failure," said Richards, whose agency handles sewage for the Ross Valley, including Fairfax, San Anselmo, Kentfield, Greenbrae and San Quentin State Prison. "We don't know how much spilled or the exact cause until we dig it up and take a look at it, but our hunch is that the two are related."

Richards said an estimated 740,000 gallons overflowed Friday in several locations in Kentfield, the worst of which was at Stadium Way and College Avenue. As much as 95 percent of the stinking brew flowed out of a manhole cover into a storm drain and emptied out into the creek, he said.

Workers dispatched to the scene found what Richards said was "an extraordinary amount of debris in our pipe."

The 30-inch-diameter PVC pipe was clogged with large chunks of road asphalt, thick pieces of the kind of rubber used on running tracks, rolls of wire and even construction helmets. The junk was enough to fill a small dump truck, he said.

"There is no rational explanation for that stuff to be in the pipe," Richards said. "The only way for that debris to get inside the manhole is for someone to open the manhole cover and put it in there."

Richards said the pipe failure Wednesday is a half mile away on the same sewage line. He doubts debris is blocking that section because it is a much larger pipe, but he said something that was dumped into the pipeline might have caused the damage.

The pipe that failed was made out of an asphalt-lined fiberglass substance known as Techite, a product that was used widely in the 1970s but was discontinued after it was blamed for numerous burst pipes. Richards said the district spent $6 million this past year replacing sections of Techite and intends to replace all of it, but he doesn't believe that was the reason for the pipe failure.

He believes someone either sabotaged the sewer line or a construction worker threw the debris down a manhole instead of taking it to the dump.

"It is not uncommon to find garbage and other debris stuffed down manholes, but usually its just small stuff," he said. "We have wheelbarrows full of debris. There is some malice involved in this. We just don't know who."

The sanitary district notified the FBI in addition to the Regional Water Quality Control Board, the Marin County Department of Health and Human Services and California Department of Fish and Game.