(CNN) -- The environmental group Sea Shepherd Conservation Society has offered a $10,000 reward for information that leads to the conviction of those responsible for shooting at least seven sea lions in Washington state waters.
Six protected California sea lions, an endangered Steller sea lion and a harbor seal have been found dead since December 1, nearly all of them in Puget Sound, officials said Tuesday.
Seven of the animals had bullet wounds, and circumstances regarding the eighth were a "bit ambiguous," said Brian Gorman, spokesman for the Seattle fisheries unit of the National Oceanic and Atmospheric Administration.
No arrests had been made by Tuesday.
"We are really saddened by this violence," Brenda Peterson of the group Seal Sitters told CNN Seattle affiliate KIRO last week.
A sea lion found off West Seattle had a bullet in one of its lungs, Gorman said.
An officer for NOAA, which is working with the Washington Department of Fish and Wildlife, showed CNN affiliate KING one of the slugs taken from an animal.
The agency will conduct more X-rays of the remains this week.
Gorman said it is not clear whether the bullet wounds were all fatal, although at least two creatures had wounds to the head.
Officers are looking for witnesses.
"Cases like this are a real challenge," Gorman said. "Unless you have a witness, it is unlikely you will find the shooter."
According to NOAA's website, California sea lions are "sometimes viewed as a nuisance by commercial fishermen, and there are records of stranded sea lions with gunshot wounds and other human-caused injuries."
"Both fishermen and California sea lions are pursuing the same fish," Gorman said.
He said it is not unusual to find sea lions that have been shot, but not normally in such numbers over two months.
The fine for shooting a sea lion could be as much as $20,000.