Officials said after tests were done on duck carcasses they still could not decisively say whether botulism or salmonella is causing the deaths, the Orange County (Calif.) Register reported Tuesday.
"The duck die-off is inconclusive based on the samples we tested," Jared Dever, a spokesman for the Orange County Vector Control District, said. "Speculation in the lab is there may have not been enough bacteria in the bird. The window is still open on botulism pending the testing of a bird carcass with a higher level of toxin."
None of the ducks have died from a vector-borne disease like West Nile virus, the district said, meaning its role in the investigation is done.
"It takes the die-off out of the arena for us because it's not a vector-borne disease -- something that can be transmitted by animals to humans," Dever told the Register. "It will require further testing; only then will you have a clue about how to stop it."
Health officials warned the public to stay away from the possibly highly toxic carcasses.
"I'm hoping it will be over soon," Lake Forest resident Anne Breuer said. "We hardly have any ducks left. I'm hoping someone will figure out what's wrong with them. Now the migratory ducks are starting to come in. God knows what will happen to them."