A river otter believed to be responsible for a rare attack on an eight-year-old boy and his grandmother in the United States' Pacific northwest has been killed.

Washington State wildlife officials said the large male otter was trapped, captured and shot close to where the late July attack occurred in the Pilchuck River in Snohomish County, outside Seattle.

According to the local paper The Herald of Everett, the large male otter was tracked down and shot on Sunday afternoon.

The paper said the state Department of Fish & Wildlife originally reported last week that a mother otter with pups might have been behind the July 31 attack.

The report said eight-year-old Bryce Moser was playing on a rope swing when family members said he was pulled into the river by an otter. He was badly cut and bruised in the attack.

When his grandmother Roxane Leilani Grove rushed into the river to pull him away, the otter attacked her, injuring her eye.

Captain Alan Myers told the paper the male otter was shot after reacting aggressively to tracker dogs.

"We have determined this is the otter responsible for the attack” based on his aggressive behaviour, he told the paper.

An autopsy has been carried out on the dead otter to determine whether it had an illness or injury that might have caused hyper-aggressive behaviour, Captain Myers said.

Otter attacks in the US are considered rare, and the July attack was particularly vicious.

Though river otters are common in Western Washington, wildlife experts say they tend to avoid humans.

Traci Belting, a curator from the Seattle Aquarium who works with river otters, described them as fast-moving, flexible and agile creatures.

"There are river otters everywhere, and people may never see them because they can be pretty elusive," Ms Belting was quoted as saying.

ABC/wires