Federal officials shot the male otter near the place on the Pilchuck River where the attack took place at the end of July, about 30 miles northeast of Seattle, according to Capt. Alan Myers of the Washington Department of Fish and Wildlife's police force.
Officials said they're nearly certain they got the right otter because of his unusually aggressive behavior. Otters are more likely to avoid people than to attack them.
The investigation aided by otter experts from the U.S. Department of Agriculture Wildlife Services was originally focused on female otters, because they suspected a mother protecting her pups may have attacked someone she perceived as coming too close.
When wildlife officials "hazed" female otters in the area, however, the animals left the popular recreational area.
"This very large male could not be persuaded," Myers said.
The otter that was shot Sunday afternoon with a .22-caliber pistol with a silencer was at least 4 feet long, officials said.
A necropsy was being conducted to find out if some disease or injury may have contributed to his aggressive behavior. Although the otter will need to be tested for rabies, the diseases is all but unheard of in otters, Myers said.
The 8-year-old boy who was attacked needed stitches for bites, and his grandmother was at risk of losing an eye from the attack near Lake Connor Park, which is between Lake Stevens and Machias.
Both have been released from the hospital, and Myers said the last he heard they were recovering.
Myers said it's safe to go swimming in the Pilchuck River again, but people should always remember that interactions with wildlife can be dangerous.
"It's important that people are always aware of their surroundings. When they see wildlife, try to avoid it as much as possible," he said.