SEATTLE (AP) -- State and federal wildlife agents are trying to trap the otter that attacked a boy and his grandmother as they were swimming in the Pilchuck River about 30 miles northeast of Seattle. Because it has pounced on people, the otter will likely be killed unless it is a female with pups, said Capt. Alan Myers of the Washington Department of Fish and Wildlife. "We're doing everything possible to capture than animal as soon as possible," Myers said Friday. "This is an extremely rare incident. Otters are not known to be aggressive toward people." The boy needed stitches for bites, and his grandmother could lose an eye from the attack Thursday morning in the river near Lake Connor Park, which is between Lake Stevens and Machias. A state worker was unable to find the otter, so expert trappers were called in from the U.S. Department of Agriculture Wildlife Services, Myers said. They'll try to trap it with a foot snare or tranquilize it. "We're not sure if it's a hyper-aggressive male protecting territory or a female protecting its young," Myers said. "There's a lot of liability in trying to relocate an animal that's been hyper-aggressive, and it would likely be euthanized. If it's a female with pups, there's another possible outcome if we can find a good location." But the otter would have to be killed in any case so its brain could be tested for rabies, if that's a concern for doctors treating the boy and his grandmother, he said. "There are no known cases of otter rabies in Washington," he said. "That's an extremely remote possibility." The grandmother was treated at Harborview Medical Center in Seattle and the 8-year-old boy at Providence Medical Center in Everett, the boy's mother, Tabitha Moser told KING. She saw the 4-foot otter attack her son and her mother saved him. "All of a sudden I just heard him scream for his life. He was just bobbing up and down in the water and as he came up there was something all the way on top of his head," she told KING. The otter continued to attack as they left the water. "Even after it got into the river and out of our way it stood on its hind legs looking at us like, 'Don't do it again; don't come in here,' " she told KING. Myers said he has not heard of any other otter attacks in Washington but there have been some around the country. Lake Conner Park has been closed, and signs have been posted along the Pilchuck River warning people to beware of the otter, Myers said.