A U.S. Department of Agriculture report released Tuesday indicates that no detectable levels of rat poison were found in the livers of five pilot whales stranded on Kauai in October.

Two whales washed onto Kalapaki Beach, and three others were found floating in Nawiliwili Harbor on Oct. 13, leading some to finger the rodenticide diphacinone, which was used by the state in three aerial applications in August and September to rid the island of Lehua of invasive rats.

But no poison was observed in 15 liver samples taken from the whales shortly after they were moved from the beach, according to the USDA report.

Officials so far have been unable to determine a cause of death, and additional testing continues, they said Tuesday.

“As conservationists committed to preserving wildlife, we were deeply saddened by the October pilot whale mortalities,” said Patty Baiao, Hawaii program director for Island Conservation, the nonprofit organization that assisted in the rat removal project.

“We are heartened by today’s announcement, which reaffirms the body of science suggesting rodent removal projects can be done with little to no risk to marine mammals,” Baiao said in a news release.