(CNN) -- A team of four NOAA veterinarians and a killer whale specialist from SeaWorld, San Diego will begin necropsies Tuesday on two wayward killer whales whose carcasses were found in Alaska's Nushagak River, according to NOAA.

The two whales that died and another that has not been spotted since the weekend had sparked concern from scientists who said they had been seen 30 miles up an Alaska river where they normally wouldn't have been.

Marine mammal scientists from the National Marine Fisheries Service said the whales were likely suffering stress from being in fresh water for such an extended period. The scientists worried if the whales didn't head downstream soon, they'd be trapped in the river.

Water levels are dropping as colder temperatures reduce the flow from glaciers into the river. That could make it difficult for the whales to navigate certain sections of the river. And the Nushagak could freeze over by the end of October, according to the fisheries service, part of the National Oceanic and Atmospheric Administration.

NOAA biologist Barbara Mahoney said killer whales are sometimes seen where the Nushagak empties into Nushagak Bay near Dillingham, but none had ever been reported this far inland. In fact, this is the first time killer whales have spent a prolonged period of time in an Alaska river, according to NOAA.

Officials said the orcas are in an area where they are unlikely to encounter humans, but they are asking that people stay 100 yards away for their own safety and that of the animals.

The fisheries service, the Alaska Department of Fish and Game, and the local Bristol Bay Native Association are monitoring the situation to determine if and how the whales could be returned to salt water, federal officials said.